My advice, start slow. If you go crazy and wear yourself out or make yourself sick in the first couple workouts you won't want to do it again. Start so slow that you are embarrassed because you know you can do more then this. Then each week add a little more.
My first time riding a bike as an adult was about 10 years ago. I got on one, and only went around the block. I basically fell off the bike when I got back to the house, my legs were like jello I could barely walk into the house to collapse. It was only a 5 minute ride. So chear up! You're not alone. We've all been there.
Maybe begin with walking 15 min a day or 3 days a week and then start to add time and distance?
http://u.about.com/
This is a good site to sign up for daily emails
Let's get walking "Let's get walking. This daily course will get you walking. The goal is to be able to walk for 30 minutes or more, 5 or more days per week. "
And then you can do the next one:
Walk for life
"Walk of Life 10-Week Program. You will receive a daily email with a walking assignment, walking lesson, nutrition tip, healthy recipe, and motivational tip. Each day is also on the web for review and printing. Let's get moving! "
Sarah