Honestly, I started cycling when I weighed about 286 pounds, I could barely go 10km without almost dying of exhaustion. Now I'm down to 163 pounds and every aspect of my life has gotten better, REALLY BETTER.
The key, I think, is to take it slow. Most people want to run 20km in their first time, they want to radically change their diet, cut out all the bad food, but it's like waging a war against your own brain and bro...you're not going to win a battle against your brain, it's too agonizing.
You take it slow, and you will win FOR SURE.