The Warriors

After each story mission, you will find yourself at The Warrior's hideout. On the lower level is a small gym area where you can increase your stats to some degree. There are 4 stations: the chin up bar, a sit ups mat, a push ups mat, and a heavy punching bag. Each of them require a certain amount of reps within a time limit - the amount of reps increase for each new level of training you complete along with an appropriate change in time available to do it in. The Heavy Bag requires you to enter combat combo strings, these are generally quite easy to pull off, even in the later levels you can still miss a few combos and still have time to complete the required reps. The Chins up/Sit ups/Pull ups are a button mash race - tap the button as fast as you can to complete the reps - these get a little harder in the higher levels, but are certainly achievable, but even if you fail any of the exercises, you can immediately restart them without penalty.

I'd recommend that you complete them between each mission, or you can wait until the end of the game to complete them all one after the other, but your character will tire after a few levels and you'll have to wait.

Completing the first 5 levels grants you increased Stamina and completing all 10 increases the distance you can sprint.