I've been lurking for a while and thought I'd post in response to some comments about thermarests.
I
love my thermarest! I've never done any bicycle touring but I use mine for backpacking, canoe camping, and sea kayaking.
Some people have mentioned a problem with sliding off the Thermarest. That's less of a problem with the newer models because they have a less slippery surface (statek I think they call it). My wife's has it and mine doesn't and I can definitely see a difference. I've also heard of people applying tiny dots of SeamGrip all over the surface of the mattress to address this problem. SeamGrip, by the way, is the best tent seam sealer I've ever found and it also works well for repairing holes in Thermarests. Also, I find the slipping off to be less of a problem if I squeeze a little air out of it after it is fully inflated. I'm not a big guy though, if you are that might cause other problems for you.
Regarding the length, I chose the 3/4 length and put my crazy creek chair under my feet. That way I can more easily justify the added weight and bulk of the chair.
