The key here is if running encroaches into your cycling training program. In the off-season, if you're not cycling as much, then running is a pretty good bang-for-the-buck aerobic activity. If you're focused on cycling as a discipline and train consistently for it during the season, then a lot of running, for obvious reasons, can be a detriment. A good training program for cycling involves a balance between focusing on different metabolic systems, progressive overload and recovery. Run too much and it throws a wrench into that whole plan.
Alwin's right about the soreness. Running is a higher impact, eccentric muscle movement. Muscles elongate slightly and stretch as your leg impacts terra firma. This causes slight muscle tearing. Cycling is a concentric movement. Your muscles are shortening and don't stretch. It is also is not a load bearing exercise. It's for this reason that adding some weight training and running in the off-season is a good idea, especially if you're older. Cycling doesn't stimulate bone growth like higher impact activities, and a lot of guys who have been putting in a lot of miles over the years now have the bone density of osteoperotic older women - The Glass Collarbone Syndrome.
If Alwin keeps crashing his new Felt at the Como Street Ride, he might suffer a broken collarbone.