Originally Posted by
Phil_gretz
Run stadium stairs
Run obstacle courses
Row a scull
Burpees
Wind sprints
Spar (but you already know this)...
None of these have anything to do with a bicycle, you'll find.
^ This. Bicycling is really not the best supplementary exercise if your primary thing is jujitsu or other martial arts. Running is a better simulation of the cardio needed for sparring, and you'll save a lot of time & money.
Most of the responses here, naturally, are suggesting you will ride more & more as you get into it, and grow out of the bike. Unless you have a lot of free time, or give up martial arts, I don't think that will happen.