I am trying to make a game and I want the characters to look somewhat like they could actually be someone who goes around slaying monsters, fighting evil over lords and their henchmen, all while still being able to hike several miles back to the nearest village. I also don't want to over sexualize the characters for sake of realism. So the only things I am looking for are weight and muscle mass. I also want them to look like fighting monsters is something they do all the time, in other words a part of there every day life.
There isn't one.
Beyond the fact that they would be in reasonably physical shape different people will have different forms and appearances based on the sorts of training they have done.
For example some people will favour agility and speed.
Others brute strength.
Wizards probably don't spend much time lifting weights.
People (particularly in a standard fantasy environment) are unlikely to be heavily overweight but other than that pretty much anything goes.
Lean but strong
Realistically these characters would be like hunter / rangers
Monsters are naturally stronger than humans so they'd kill them using arrows and/or traps. Evil overlords have more men and are better armed and armoured so again it would be ambushes, traps and arrows from afar.
You don't want bulky muscles because it's extra weight hiking back and extra calories to keep up the strength. Since you're using arrows against a stronger opponent, you don't want swords nor carry the weight of a sword while retreating back to cover. Equipment would likely serve dual purpose and a legitimate use to explain to passing patrols. Hunters and woodmen need bows, daggers and axes but a sword is hard to explain.
It doesn't matter. Your only listed requirement is cardio (long hikes). You can have a strong heart and be any size and shape. 300 lbs people run marathons. 70 lbs people can't do a single push up or walk more than 5000 steps without becoming exhausted. So it really doesn't matter what someone looks like, it is what is inside that counts.