Yeah, I think challenge can be a bit motivator for adhd folks. Once I've completed the main part of something, I find it really hard to care about the details, to the extent that the unfinished parts sometimes spoil the bit I had completed.
I feel like it's the dopamine of the chase is actually what's motivating, and challenge is a version of that. I'll get sucked into finding some obscure game and getting an emulator working to be able to play it and all the way I'm super engaged. Then I start playing this game I was so excited about and meh, don't care.
Maybe you could think about ways to refocus that drive? A therapist told me once that adhd people don't get satisfaction from completing things, but are excited about new things. So, instead of feeling proud of getting into college try and immediately find the new challenge (now I want to get a prostigious internship!) if you succeed at your fitness goals, maybe you can raise the stakes see if you can beat a friend or a record or something?