Echo what everyone else says. I also have a 5 mile each way commute, and when I started, I was winded by the end of it. I noticed a few months later that I no longer was. (It probably stopped happening long before then, but I only noticed it then.) That was 3 years ago.
Recently, I had occasion to try a Very Long Commute ("VLC") for 3 days in a row, a 34 mile round trip. I was amazed that not only could I actually do it, but I felt like I could have gone even longer if I had to. I think I have reached the point now that the limiting factor on whether I bike somewhere is not whether I can physically do it, but merely if I can afford the time it will take. It's a good feeling, and I'm sure you'll get there too, if you keep at it.
Good for you!
