Usopp walked through the forest and tried to keep up with Luffy and ace, but unknown to him Luffy and ace have been trained since young and their physique isn't something a boy who never trained before could match. After 30 minutes he fell to the ground and watched helplessly as they ran off leaving him in the middle of the forest.
Usopp caught his breath and fell into a deep thought, 'If I can even catch up to him how can I ask him to train me?' After that he walked back to the cabin with his thoughts racing, 'If I'm gonna be strong enough to be able to sail, I'll have to be able to do more than just fire a gun at someone.'
When he reached the cabin he immediately asked Dadan where his room was and began writing out ideas on how to get stronger and worked through the day until it was night. So as the night passed, Usopp woke up and began running through the mountain at a brisk pace. He continued until he felt his legs give out all while keeping his dreams in his mind for perseverance.
After running, he ate whatever Dadan would feed him and continued his workout with his upper body. He did push-ups, sit-ups, anything that could help increase his strength. He continued this everyday for the next month, not noticing Ace watching him every time he left. Luffy would try and talk to Usopp every time he came back from chasing Ace, but Usopp wanted to keep his mind focused and told Luffy politely that he wouldn't be able to play because he was training. This of course made Luffy want to join, but he never was someone to have a good attention span unless he was fighting or eating.
Eventually the month rolled by, and he felt satisfied enough to run through the mountain without passing out. So the next day, surprising Ace and Luffy, Usopp followed them. When Luffy asked about it, Usopp said he'd gotten to a point where he could at least follow them and not pass out so he wanted to follow. Luffy laughed and was happy his friend would be joining him in chasing Ace down.
As they continued through the forest, trying to dodge everything Ace was throwing at them, they finally reached a bridge crossing a mountain valley. While Usopp and Luffy were crossing the bridge with heavy breaths, Ace was standing at the other side of valley with a cold look. Once they were halfway across the bridge, Ace started walking towards them. Luffy started smiling obviously happy that Ace was going to talk to them, while Usopp just watched and started to get tense when Ace gripped his pipe tightly. When Ace was just in front of them, he swung his pipe. Usopp ducked it just in time, but Luffy was hit and fell down the valley. Usopp got up angry and swung his fist as hard as he could towards Ace. He dodged and hit Usopp with his pipe as well, but he only hit him back towards the bridge.
"What the hell is your problem!" Usopp screamed, picking himself up. "All he wanted was to be your friend and you hit him off a bridge, you could kill him."
"Why should I care, it's not like I wanted him to follow me." Ace replied coolly.
Usopp was seething in anger and thought back to the Kreig pirates and the marines that visited his island. 'People just do what they want and don't care about the consequences to their actions. They're all the same, the only good people I've met were Garp, who dropped me off with a monster for a kid and mountain bandits, and Luffy, who's too stupid to know when to trust certain people.'
Usopp got up, walked back towards the other end of the bridge, and started scaling the mountain.
"What are you doing he's dead." Ace said.
"What's it to you, he ate a devil fruit, theirs a chance he's alive. Plus, he's only friend I've got." Usopp replied angrily.
Ace thought nothing of it and walked off while Usopp climbed down the mountain. Once he reached the bottom he searched all around to find Luffy.
"LUFFY!"
"SAY SOMETHING!"
Eventually he found the boy with one leg trapped under a rock. "Luffy," Usopp yelled and ran towards him. Usopp immediately started pushing the rock off of him and finally got his leg free. Luffy didn't say anything and just looked down, tears dripped down his face. "Why doesn't Ace want to be friends with me." He said in between sobs.
Usopp felt bad and said, "I don't know Luffy, but why do you want to be his friend after all he's done. I'm assuming this isn't the first time he's done something like this."
Luffy looked up with tears in his eyes, "You and Ace are the only people I can be friends with here. It's much better to put up with this than to be lonely."
Usopp faltered for a second, with a tear rolling down his face he said, "I'll be your friend Luffy, and we'll keep trying to get Ace for you." Usopp personally didn't care for Ace, but that didn't matter as long as it made Luffy happy. Usopp had never had a friend either, even in the village he was only known as Usopp the Liar. Now that he's decided to cast off the moniker and become someone his parents could be proud of he wants nothing more than to have good friends.
Luffy looked at Usopp with a smile and said, "Un."
Usopp sprouted a smile himself and said, "Now we have to figure out how to get back to the cabin."
After that, both boys made a long trek through the woods, constantly running into wolves and bears and running for their lives. Two days later, luffy and Usopp stood outside the cabin with a dog barking at them as Dadan stared at them. They were covered in dirt and scars up and down their bodies. Dadan picked them both up and through them into the room with Ace. Once they hit the floor they both immediately passed out from tiredness and hunger. Ace woke up to the noise and was startled to see them both alive, but he thought nothing of it and went back to sleep.
For about a week after that incident, Luffy and Usopp didn't follow Ace. This bewildered the young boy but made him happy nonetheless. Now he didn't have to worry about this deadweight following him.
The next day, Usopp could be seen huddled up with Luffy, "Alright, you ready Luffy?" Luffy nodded to Usopp who continued, "We've held off for a week to build our strength back up and lure Ace into a false sense of security. Now we follow him, only this time we're gonna track him instead so he doesn't see us."
Luffy nodded again and we got to work. We followed his usual trails and finally after two hours came across a giant tree with two figures sitting inside.
"Alright Luffy, now that we've found him we are gonna continue to obser-"
Before Usopp could finish Luffy was racing to the tree and stood below it. Usopp tried to whisper, "Don't!" But luffy didn't hear him and yelled, "So this is where you've been going to Ace!"
Usopp froze and sighed, "Luffy, you idiot! How I'm I supposed to get you out of this now."