Keep going, Dan. You can do it. It just takes time and perseverance.
I started riding in 2007 at 320 lbs. I had been a football player in college and while big, had always considered myself "healthy" and never worried--or thought--about my weight.
At a later check-up the doc had said my blood pressure was "getting up there". I couldn't believe it. Even though big, I had always had good blood pressure. The second check resulted in the same. So, prideful that I am, I started riding again. I had ridden and raced from 1987 (after football) to 1997 before I got distracted and ballooned up to my 320 lbs. I had been a somewhat experienced/accomplished cyclist. I had done centuries (mountainous ones), 35-mile road races, time trials, you name it.
My first ride (in 2007) was down the local bike path 5 miles and back. Totally flat. It took me an hour. Hurt like h*ll. And it was very humbling.
Since then, I've accomplished some amazing things on a bike, even more than before. I'm currently at 260 lbs. and holding steady for a couple years. My blood pressure is back to nice-n-low, and I consider myself healthy once again.
And now for what you may be interested in: my riding is at a point where very few centuries interest me anymore (I can drop what I'm doing and go do one pretty much anytime). I ride 200+ miles a week and annually put in 8-9,000 miles. I've ridden a flat century in 4:05 and a hillier one in 4:50. And I've completed the SoCal KOM series (boy that was hard!). But what I enjoy the most is the fun and camaraderie with friends while I'm riding, accomplishing goals, and the adrenaline "high". And now, my 15-year old is into riding and that has me excited.
As I said before, you can do it. Not trying to toot my own horn above, but more: "If pathetic-dude-me can do it, anyone can do it!"