I was the exact same size as you when I started commutting on my new bike last year 11 months ago. I only have 5 miles to work one way and I do that 5 times a week and 60 miles on saturday (30 miles each way part time job). I looked at the short distance as a sort of time trial and constantly try to beat my last time. I also watch everything I eat and take vitamins from gnc along with tri-flex for the knees. I have lost 20 pounds with effort but I also never slack off which is hard with 3 small kids. The way I did it was push myself when I thought I couldn't like going to 20mph from 18 or 22mph from 20 an so on. You can get faster its all in the mind!