I know you mentioned walking and I highly suggest it. I started out at 345 lbs on 1-1-09 and a year and 4 months later am down to 169. I got a used treadmilll- nothing fancy and started out slowly. When I say slowly I mean like 5 minutes at about 2.5 miles an hour. The more I lost the easier the walking got and the longer I could go. I can pound out a mile or two in no time at all now.
If you don't have access to a treadmill you can always walk outside. Throw the headphones on and go. I used to drive around the parking lot looking for the closest spot. Now I park far away and get in those few extra steps as everything counts!
Good luck with your weight loss. Keep your mind in the right place and you will succeed.