Originally Posted by Ciufalon

My approach would be a little different. I suggest going at a slower, comfortable pace so that you enjoy the commute and your endurance will naturally build up. You get stronger without realizing it and just naturally go faster as you become more capable. I speak as a both a long distance runner and a cyclist. I know that for running fast I run slow! That's right slow, occasionally very slow. It is called an aero run and it causes your body to create more/new capillaries, the tiny blood vessels that carry oxygen to the muscles. By just going at a comfortable, aerobic pace, some days you will feel stronger and feel like pushing yourself. On those days do it. Also, if you start the ride/commute out at a slow pace - slower than you would want to go and try to hold yourself back for a few minutes, you will find your body just naturally speeds up to a good aerobic pace that will feel like you could go on forever. Don't make things harder than they need to be. As for spinning faster in in lower gears, well, all I can say is that when I do that my legs are trashed immediately. Ever watched the folks in a spinning class? For the distance you are riding, I would say forget it. A long distance runner does not run fast every time. Generally, only rarely when training and on race days when they are going for a personal best or a specific time. Commuting is not racing. Enjoy the commute at a pace that is comfortable and allows you to get in better shape without even realizing it. I got back into cycling years ago because I remembered that I rode a bike all over the place as a kid and was in great shape as a result of getting exercise without realizing it; while having fun. That is how I get all my exercise - doing things that are fun; not working out! Few people stick with a miserable workout routine. Most quit after a short period of time, because it isn't fun.

Other may have different opinions, but this is just what I suggest.