You choose a class (endurance, strength, or something in between). You have items and skills which you have to skill up individually (barbells, dumb bells, gymnastics equipment, running, cycling, etc.). The same dilemma exists for leveling up: skill points added to one category are skill points you can't add to another category. Thus if you split your time between strength and endurance you will never achieve as much as someone who exclusively trains one of the two.
The skill curve is even similar to that of a well designed RPG: in the beginning you make a lot of gains quickly but eventually you slow to a halt and it takes months to even gain a few skill points in one item/skill.
The gym in real life is like hardcore mode: If you don't play for a while, your skills degrade as a penalty. However, the skill lost as a penalty is easier to reattain then skill you never had.
There are even boss fights and co-op mode if you decide to compete.
There are too many parallels to list them all here, but I think you get the idea.