I'm somewhat new to cycling, having been a swimmer for around five years now. I find that I actually max out on volume much quicker than intensity in cycling. I think that since I swim short events, usually the 100, 200 and 500 short course yards, I,m used to the cardiovascular load of intensity, but my body just still isn't used to the long-term stress of hours of biking. I just don't do those kinds of high volume workouts when I swim, so for me, too many long endurance rides will leave me fatigued.