I have a 20 mile round trip commute and I make the best of it. I only take it easy over the last mile or two on the way in, and I'm in Manhattan traffic anyway so you can't really do much. Even going home though I go as fast as I feel safe on the streets. About half the ride is off the streets and that is where I really push it. Some of the other cyclists think I'm a bit crazy, this old grey guy pushing it up the bridge, barely able to catch his breath. I try to keep my HR below 170, but I've hit 185 going up.