Rayleigh's training mostly focused on haki, with sword duels thrown into the mix. As for physical training he gave up after the first day when Marcus asked, "Is this the warm up?"
"Warm up? No… This is the best way to help increase your physical power."
Marcus adjusted the large boulder on his back as he asked, "The weight's not bad, but what else are we doing with it?"
"That should be just enough weight for you to carry without collapsing, there is nothing else to do besides carrying it till you can't…" Rayleigh said in a confused tone before he took a drink from his flask.
Marcus frowned as he adjusted his footing before he started to do squats with the boulder and said, "My grandpa made me do something similar, but he always incorporated exercises into the routine to make it as efficient as possible."
"Only a madman like him would come up with something stupid like that."
Marcus shrugged and continued doing squats as he said, "I've been doing stuff like that since I could walk…"
Marcus started to laugh before he said, "Actually, the first time gramps ever trained us was right after we could walk a handful of steps."
Marcus switched from doing squats and adjusted the boulder so he could do push-ups with it on his back before he asked, "You know what he told us before he dropped us next to a gator nest?"
"He did what?!?"
Marcus ignored the outburst and continued, "He basically said, If you can walk, you can run. I have no idea how long the training lasted, but being chased by a gator is one hell of a motivator and we could easily run by the end of it."
"He's definitely a madman… Who let's children be chased by gators?!?"
Marcus was doing two finger push-ups as he said, "Someone who wants his grandkids to be safe and considering I can already handle the first half of the Grandline with ease… Well, I think he did a damn fine job."
"If he was doing such a great job, why the hell did you come to find me?" Rayleigh asked after he took another drink from his flask.
"He only comes by once a year at best and can only stay for a few weeks to a month. Kinda hard to get proper haki training like that… Plus gramps doesn't know how to use a sword." Marcus said as he stood back up and started doing squats again as he added, "I could have stayed and trained more against my brothers, but from my understanding haki training works best against stronger opponents."
Rayleigh nodded along as he said, "You're not wrong. Haki, especially conqueror's haki, grows fastest in fights against stronger users."
"Which is why I sought you out. Besides Shanks, you seemed like the best teacher for me."
Rayleigh rubbed his chin as Marcus started doing jumping jacks before he said, "You're not wrong, but I'll only train you on swordsmanship and haki going forward. You can do whatever you want for the physical side of training since you already know what to do…."
"Sounds good to me." Marcus said before he stepped in front of an out cropping of rocks.
Marcus adjusted the boulder tied to his back before he took a stance and then punched those rocks without haki. Rayleigh's eyes widened as he shouted, "What the hell are you doing?!? Are you trying to break your hand?!?"
Marcus pulled back his fist and adjusted his feet before he punched the rocks with his off hand as he said, "I'll be fine, gramps has had me doing this type of training since I was a kid."
"You're still a kid!" Rayleigh said in an exasperated tone.
Marcus chuckled before he said, "Oh yeah… Well I've been doing training like this for at least six years. Started with trees and worked my way up to harder things like this over the years."
Marcus struck the rocks a few more times before he added, "Gramps said once I get strong enough, he'll let me punch battle ships at some marine shipyard. Once I can move one of those ships with my punches, he'll give me the final training test which is punching a mountain into dust."
"Garp's gonna teach you his signature move?" Rayleigh asked in concern.
Marcus chuckled as he said, "While I would love to learn that… The goal would be to do this type of training to an entire mountain and slowly destroy it in the process."
"You're just as crazy as your grandpa."
"Thanks!" Marcus said as he punched the rock again and it cracked this time.
Rayleigh took another long pull from his flask before he said, "Don't exhaust yourself with training and come find me once your haki has recovered so we can train it again."
Rayleigh left Marcus to his physical training after that and only occasionally watched him when he was bored. Except every time he found Marcus, he was doing something incredibly dangerous and would just say it was all part of Garp's standard training.
The first month passed in a flash and Rayleigh's haki training was giving Marcus the biggest boost so far. He could last almost an hour in their spars at this point and his conqueror's haki was a lot more powerful then before. It was still nothing compared to even Doflamingo's, but Marcus wasn't worried since he just turned ten and he doubted most people's conqueror's haki could match his at the same age.
As for his swordsmanship, it was on the cusp of a breakthrough. Marcus could feel it in his bones that he was just missing something…
Everytime he picked up his sword and used it, he could feel a barrier holding him back. Something inside himself was telling him that how he wielded the sword was off, but wouldn't tell him what it was. Marcus tried to probe his sword for the answer and found nothing. Even Rayleigh was no help and told him, "No one can help you past that barrier. You either get it or you don't…"
Rayleigh took a long pull from his flask before he stared out at the calm waters near the island as he said, "The difference between a truly great swordsman like Mihawk and a decent swordsman is that barrier. Once you break past it, your growth will explode… But only if you can break that barrier."
Rayleigh turned towards Marcus who was in the middle of practicing his sword swings when he added, "If you're lucky you'll break that barrier during training. If you're not, you'll break it in the middle of a life and death duel. And if you're unlucky…"
"I'll never break past the barrier?"
Rayleigh shook his head as he said, "No, you'll most likely die trying."
Marcus performed another overheard slash as he said in a sarcastic tone, "I appreciate the vote of confidence."
"I'm just being honest… I've seen too many swordsmen die chasing that goal and just want you to be aware of what you're actually after. It's a path few ever finish and it's paved with the bodies of everyone who's ever tried."
Marcus performed another slash before he said, "It wouldn't be worth doing if it was easy."
Marcus raised his sword over his head again as he continued, "Besides, who wants to live a boring and easy life?"
"Well said, kid." Rayleigh said as he raised his flask in a toast before he took another drink.
Rayleigh jumped off the tree branch he was on after he put his flask away before he picked up the stick he liked to use and said, "Come on kid, let's have another spar."
A couple more months passed as Rayleigh increased his end of the training. It was mostly in sword duels and he didn't hold back like before. Whenever they fought, Marcus would be injured, sometimes it was serious and it would take days for Marcus to recover properly. Sure it affected his training somewhat, but the growth he gained afterwards more than made up for the lost time.
Marcus's haki also showed improved growth compared to before and he could now fight for almost four hours before he was exhausted. As for his conqueror's haki, the improvement was decent but Marcus knew he was still a long way off from reaching Doflamingo's level which was his short term goal.
The training continued and around the ninth month or so, Marcus received a phone call. Since he started training with Rayleigh he'd only made a handful of calls. All of which were made just to tell everyone he was fine and would be training for at least the next year. Since then Marcus didn't carry around his den den mushi as he trained, so he probably missed plenty of calls. This time he was in his room resting after Rayleigh sliced open his stomach during a duel.
As the den den mushi started to ring, Marcus instantly picked it up and said, "Moshi, moshi, Marcus speaking."
"Brat! Do you have any idea how many times I've called you?!?"
Marcus picked his ear as he said, "No clue, but I told you I was training and probably wouldn't pick up if you called."
"I'm your grandpa, you should always pick up when I call!" Garp said in an annoyed tone.
"That's hard to do when I don't have my den den mushi on me…"
"You little brat… Tell me where you are." Garp said in a tone that promised violence.
"Why? Are you on vacation?"
"No, but if you're near Marineford I could disappear for a day or two to visit." Garp said before someone in the background said, "You can't leave, we were called in for the meeting tomorrow Garp-san."
Marcus raised an eyebrow as Garp said, "Bah, that meeting doesn't matter. I'd rather beat up my grandson."
Marcus started to sweat at Garp's statement and thought about hanging up until Rayleigh's voice carried over, "We're on an island in the Calm Belt Northwest of Impel Down."
"Perfect, I know where that is. I'll see you soon, brat." Garp said before he hung up.
Marcus glanced at Rayleigh and let out a sigh before he said, "I know my grandpa seems to have a relationship with the Roger Pirates… But he'll probably try to fight you if he sees you here."
Rayleigh chuckled slightly and took a swig from his flask before he said, "Maybe, but most likely we'll just clash for a round or two. Me and Garp are too old to seriously fight each other."
"My grandpa fought with Zephyr in a friendly spar years ago and ended up with a broken arm for a few weeks… Zephyr was slightly worse off."
Rayleigh was mid drink before he spit it out and said, "What the hell is wrong with him?!?"
Marcus shrugged as he said, "It's Garp, trying to understand his thought process will just give you a migraine."
Rayleigh lifted his glasses and pinched the bridge of his nose before he said, "Alright, I'm taking your ship and leaving for a week. I'll be back once he's gone."
Marcus frowned as he said, "And what if you wreak it?"
"It's a single man ship, I can handle it."
Marcus thought about it for a few moments before he said, "Fine, but let's unload my stuff before you leave… Just in case."
Rayleigh agreed and within half an hour, all of Marcus's personnel items were on the island including his guitars, amplifier and speaker. Clothes, log poses, sword, blue prints for his ship and a few other miscellaneous things. Marcus waved goodbye to Rayleigh as he started to pedal the drive train and he quickly engaged the hydrofoils before he disappeared from view. Marcus stored all his gear in a small hut they made on the island before he strung up his hammock and jumped in it to take a nap.
Marcus was completely asleep when a marine battleship with a giant dog's head on its bow appeared on the horizon hours later. The ship anchored next to the island as Garp jumped off of it and headed towards Marcus. Instead of a normal greeting, Garp's fist slammed into Marcus's head which broke his hammock and buried him like a lawn dart with his head stuck in the ground.
"Get up brat, I only have a day to train you."
Marcus struggled with the way his body was stuck in the ground but after a minute he was able to pull himself out before he spit out a mouth full of dirt. He stood and dusted himself off before he shot a glare at Garp and said, "What the hell old man, is that how you greet someone?!?"
Garp raised his fist menacingly as he asked, "What did you just call me, brat?"
"I called you a senile old man who's probably been hit in the head one too many times!"
"You stupid brat!" Garp roared as his fist descended towards Marcus like a meteor.
Marcus pushed all his strength and haki into his own fist as he countered Garp's attack with his own. The sound of a massive thunder clap rang out that almost made Marcus go deaf before Garp's distinctive laughter filled the air.
"Bwahahahahahahaha, That almost tickled you little shit!"
"Oh yeah? Well try this old man!" Marcus said as he flared his conqueror's haki to his current limit and threw another punch towards Garp.
Garp flared his own conqueror's haki and it instantly crushed Marcus's as he countered with his own attack. Another deafening thunder clap rang out as Marcus was sent flying by Garp who roared with laughter before he chased after Marcus. Over the next hour Garp proceeded to 'instruct' Marcus on how to properly fight with conqueror's haki using his fists. By the time it was over Marcus was one giant bruise and Garp was roasting sea king meat over a bonfire.