You mentioned in a couple of posts that you have a mountain bike. Did you ever think of using that for your commute, at least on days when your legs are fried? I have an old hard tail Fuji that I use for my winter commuter. It's a lot slower than my usual commuter, but put it in the granny gear and you can ride it up a wall. It also has the advantage for your self esteem - nobody expects someone to go as fast on a mountain bike.