I seriously considered creating a new ID before doing this post. Yes I am a closet Tri geek. (Tom Cruise will you please come out of the closet!)
I did a group ride with my Tri friends and discovered they don't really know how to ride with a group. Hence they don't. I would agree the best roadies can crush some of the best Tri guys. However, a Tri guy who rides with roadies, in addition to his/her regular training should be able to crush most challengers on the bike. If you are fast on a bike, you are fast on a bike.
Group rides are harder training than going out on a bike with aerobars for a 3 hour solo ride IMHO. My HR monitor says I work harder on group rides. That's why I try to ride with a group whenever possible.
As far as running goes, I joined a masters track group. I found the same thing. It pushes you more than running solo EVER would.
So regarding the orginal post, run, bike, do whatever. If you enjoy it, keep doing it. I do however subscribe to the math that 1 hour running is equal to 3-4 hours on the bike. Again only MHO. You get slower only if you think you do. You mind is the biggest limiter you have.