I did my first workout so far where I went directly from biking (ok, spin) to running. I thought I would be fatigued and it would be tough. But, it was actually a different kind of tough than I was expecting. Aerobically, it was fine, no biggie. But, since you use different leg muscles to bike, the running was really strange.
I did 45 minutes of spin which totally kills my quads and hamstrings and then went right into a 3 mile run. It took a whole mile for my legs to loosen up. I was really having trouble turning them over. I didn't want to run fast for obvious reasons. But, I also didn't want to just "jog" the whole three either. At the end of the first mile, my legs didn't feel as bad and I was able to pick up the pace and finish the other two miles at a nice average clip.
I'm curious though... I'm hoping that the more I do this, the less that transition affects me. If it takes me a mile to get started "running" in the race, that's gonna be a problem. I'm over 3 months away from the race though... so I've got plenty of time to find that out.