This is a very emotional topic for me. Before I got this disease, I was training for my second degree black belt in taekwondo and working out like 15 hours/week. I loved it. My husband (then fiance) and I went hiking regularly and did swing dance together on weekends.
When I first got the disease, I didn't know how to manage my energy levels well, so I couldn't workout more than 5 hours/week, and even that was at lower levels than before. I never got the second degree black belt.
During my really bad flare, I couldn't do anything, lost all my muscle tone, and was lucky if I could walk like 3 km in an hour. Finally, after being off prednisone for 6 months, learning how to manage the disease, and taking a regular multivitamin (I think it's the b-complexes that don't absorb as well with UC and those can have a major impact on your energy), my body is starting to feel a bit normal. However, since I've spent the last year + at such comparatively low levels of activity, I have to build up slow. Hiking and swimming seem to be good for me, even though I'm painfully slow compared to where I was. Yoga and walking was all I could manage first out of the hospital, though.
What I decided (and it's been working for me lately) is that 4-6 days/week I do something for an hour or two, but that I can choose whatever feels most comfortable on that day. When I'm having a low energy, stomach pain-y, or high BM day, I usually choose just walking or biking at a slow pace. When I'm having a high high energy healthy day, I take advantage of it by jogging or climbing a tough mountain.
But the disease has (so far) left me unable to "train" in a traditional sense because I can't just set a schedule and expect my body to comply. Perhaps when I've been more stable for a longer period of time and have built back more of my base strength, I can train again and maybe even return to martial arts... but for now, I'm just so grateful I could climb the mountain I did yesterday or swing dance for 2 hours a few days ago. There was a point at which just walking up the stairs at work gave me tendonitis from the strain it put on my body. I'm glad I'm past that...
Good luck to you!