I certainly find it is intensity that affects me these days. Distance/time isn't really a problem but speed kills!
Initially it was being able to do lots of training, not get injured and so actually make consistent progress in fitness that made cycling so good, plus learning new skills and racing is so totally different from running. Now I'm cranking things up more and more and its fair to say that I'm surprised how much quality / intensity I can do also. That said though I'm exploring and pushing limits so have gone over the top a couple of times this spring - each time when stepping up to the next level - did when I started L6 (anaerobic efforts) and now with Level 7 ( Neuro muscular power) Thankfully there are no more levels after 7 -