As i've gotten older I find that it takes me longer to get over sore muscles. Try a google search on "Delayed onset muscle soreness" and you'll find a bunch of stuff, some of which may even be credible. Additional protein is not one of the treatments most of these sites list, probably because the average western diet has plenty of protein in it.
Usually, as you get used to an excercise you stop getting this, though. And generally, bicycling is pretty low-impact, and I, anyway, never got _real_ sore from it. Perhaps someone with more knowledge will chime in here...