Walking is great. So is pedaling. So are the sparkpeople quick cardio workout videos. HOLY intense!!!! Seriously! You want to do what you can, especially if you're not accustomed to regular exercise, and make sure you get the clearance from your doctor before you start anything. I'm obeser than you. It's a word, now. Don't judge me.
Playing. Anyway, seriously-I'm bigger than you are, but I'm not terribly sedentary like I was when I was younger. You really have to build up your tolerance and intensity bit by bit.
I have faith in you! You can do this! Walking the pup is a GREAT way to start!