Good luck my friend. When I was 50 I weighed 360 lbs. I'm 6'2. I started riding a little and dieted down to 170lbs in about 18 months. I was riding about 100 miles a week then, but I messed up and injured myself with a double hernia somehow. I'm fixed now and getting back in shape after everything.

I wanted you to understand I know a bit about how you feel. I'll say this. You have to know you are still extremely heavy and you need to understand that this is going to be a long journey back to health. Take it slow and under a doctor's supervision. Outside the health issues you are heavy enough that you don't want to fall at any speed. Chances are you will fall at some point, and you need to know how not to break your wrists if you do take a spill for whatever reason. I've taken a few bad ones at 180lbs. It would have been much worse at even 300lbs much less 400lbs plus.

You have to consider that every component on that bike is being maxed out. There are likely much safer bikes out there. I'm very happy you are out there cycling. That's what you should be doing but I would suggest working on very slow balancing movements and slow climbs to get your exercise in. Maybe look into very heavy weight rated bikes. I don't know what's currently out there. Something like a Surly Big Dummy comes to mind. Just take it easy on the fast stuff for a while my friend. It's actually surprising how good of a workout you can get just learning to track stand in the yard and all those balance tricks that don't stress things too much at your weight. Take things slow and listen to the voice if it tells you to slow down before you kill yourself. That goes for speed or effort. You can probably lose 100 lbs. a year but listen to your doctors. Just realize this is a slow journey that you can accomplish if you don't give up or get hurt by taking things too fast. Good luck and best wishes