The only negative side to running (in tandem - pun intended) is more risk due to injuries. Echoing others posts, running is great for topping up the fitness and mixing up the exercise. I plan on throwing in a run once a week to maintain some leg strength. I joined a running club a couple of years back and made good gains within two months and met some nice people before stumbling into cycling.
Greg seems to be well into his running chipping in a bit of favouritism there I think. Personally I agree with brumjim, if I run at a 'training' pace then you cannot hold a conversation for long, and on another note I find cycling clubs just as social if not more-so than running clubs so not really with you on that one Greg but sounds like you found your ideal club there! Lastly I hate running unless I have some music to listen to, I'm also not built to do marathon distances whereas cycling I never get bored of the rides.
