If you get fat enough, it actually becomes pretty difficult to exercise. You try strapping on a 150 pound fat suit and see if you can still run an 8 minute mile.
It's not about a 8 minute mile when you start out.
I've helped two people who were obese lose a lot of weight, though they weren't obese enough to be bed ridden, they were obese by definition.
My father (6' tall) moved in with me after he fell on hard times (divorce, health issues). He had the beginnings of diabetes, high blood pressure, and weighed over 300 lbs (none of it muscle from working out). He smoked also.
When he got to me, walking even 1/4 of a mile was almost too much for him.
He now weighs 225, and some of that is some muscle. He probably lost 100lbs of fat. His diabetes is under control with pill (stopped it before it progressed to where he needed injections). He no longer smokes.
Know what I did? Got him to stop smoking, stop eating junk food, and commit to working out a minimum of 5 days a week. Started out simply walking. He lost about 50lbs WALKING ONLY. He progressed so he was talking walks that would last 2-3 hours. Then we started adding in some basic weight lifting, along with the walks.
I had another female friend who weighed 240 lbs. She now weighs 168 (last weigh in) and is continuing towards her goal of about 150. Know how it happened? A little at a time with a lot of commitment. Changed her diet, and I started coming over to her house to do the insanity DVD's 5 days a week. When we first started, I only had her do the 10 minute warm ups (so none of the actually exercise). She couldn't even complete that. But she was dedicated. Once she got so she could get through the whole warm up without needing to take a break, I eased her into the basic videos, on to the advanced, and then added in a few jogs a week.
Both of these processes took over a year. It took DEDICATION and TIME. This isn't something that's going to happen over night. But it also took ZERO medication. Just modified eating habits, and committing to physical activity at least 5 days a week.
Unless you are so overweight that you literally are bed ridden, you can start out with walking and getting your heart rate up, and modifying your diet.