Whatever exercise you thought of... that's the one you're doing.
I am giving you the opportunity to cum, slave. All you have to do is exercise enough to warrant enough strokes to cum. Refer to the following chart:
Push-ups - 2 seconds of stroking per push-up
Sit-ups - 3 seconds of stroking per 2 sit-ups
Wall-sits - 1 second of stroking per 2 seconds of wall-sits
Mountain-climbers - 1 second of stroking per second of mountain-climbers
If you can cum in the amount of time you earn through doing your hardest exercise--then you are allowed to. If you can't--then I guess you don't get to cum.
Move on only when you have either cum, or decided you don't want to today.