Now you may of done the thing were you max out you cheat by walking into a wall in sneak mode but i believe it may also work for athletics. you can either swim(which will get you more XP) or you can run/walk

1: if you run the go to a place were you can run into something and will never move if you swim find a place in water were you can move like swim into a hill you caracter cant climb or a rock in the water.(if you swim make sure your character keeps his head above the water otherwise he may die. Save if you dont trust it)

2: use a rubberband or anything else to hold down the stick that makes you walk

3: you can leave the room for as long as you want and if im rite when you come back your athletics should be maxxed (period of time it takes from 0 is unknown b/c ive never actually tried this its only a guess)