Never too fat to run!
Even if only 20 feet! If it's 20 feet without pain, you are good.
Seriously, if you haven't run in a long time start off slow. Maybe run past a driveway, then walk until you've recovered from that effort (no panting, no fatigue, no pain), run past another driveway, walk until recovered, etc.
Your joints have been dealing with your weight every second of every day. If they aren't bugging you now they are adjusted to your weight now. It's the new activity they need to adjust to. And ANYONE who hasn't run in a while should start like that! Not just someone who has extra weight at the moment.
Go for it!
Just remember, nothing should hurt. If it hurts, run even shorter differences between walks. If you just can't run at all without pain, you should really check in with your doc. (I'm assuming your doc already ok'd the tris?)
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson