sidenote spurred on by your query about running vs. cycling
-- if you're mainly cycling as your primary form of exercise, DO make time to at least do a couple of longish walks during the week... otherwise you may be prone to atrophy in various key hip/butt muscles, which aren't involved in seated-pedaling.
experts may say different, i'm saying this has happened to me, and my solution is to just make sure to do some walking!