Warning: I've noticed this exploit isn't flawless. I tried resuming workout immediately and it didn't work. I only recently discovered this exploit, so give me some time to experiment with it to see what the rules are.Here's a trick I've recently learned. I hope you all like it!
My sims often have desires to workout for X number of hours. Sometimes, using the "good pacing" workout style can help me work out for longer, but sometimes, I still get fatigued before I can complete the want. If I get fatigued, I can't workout again until the fatigue moodlet expires.
However, I've learned that I can avoid a fatigued moodlet if I wait until right before I'm about to get fatigued, then cancel the exercise, go grab a shower, and then come back. The amount of time I can spend working out restarts, even though I was only a hair away from being fatigued just a minute ago!
This obviously wouldn't help if your sim wants to work out "until fatigued," but it helps if your sim just wants to work out for X number of hours (either for a want or for celebrity points), or if you just want to grind the athletic skill.
Use this to get infinite amounts of exercise!
You are welcome!