Congrats on starting! I just started commuting in June, it's a great feeling and it's made a substantial difference in how I feel and overall health. I was hooked after the first ride.

I started by throwing the bike on the car and having my wife drive me to work for about a week. Sounds like you can ride and take the bus to work then ride the whole way home. That stamina will build up before you know it if you're putting in 8 to 9 miles/day by riding the whole way home.