I have a 19 mile commute each way, and given the time commitment, I really only do it once a week. Each time I do the ride it gets a little bit easier. Climbs that were slow and steady used to kill me - now I find that I have the energy and can push myself a bit to get up the hill faster than I used to. Where I had downhills, I used to coast to conserve energy, but now I can pedal to push the speed up a bit higher. The recovery time seems to be going down too - I am no longer as stiff and sore after a ride as I used to be. And I am noticing that the overall time to get to the office is slowly going down as I can ride in higher and higher gears.

My first commute like this was last summer - I didn't ride much over the winter (except on a trainer), so I felt like I was starting out at scratch when I started riding again in March.

Other people's comments about warming up are important. The first 3 miles or so I take really easy. When I have to climb, I will pick a low gear and spin. In my commute, I hit a MUP at mile 2.6, and then I have about 2 miles of slow gradual downhill. By the time it levels out and turns up again, I feel completely warmed up.

My own take on this is that the 3 miles you have just aren't long enough to really build up much strength and endurance. Longer rides on the weekend would help a lot, I think.