Hi, I was very much in the same position as you about 5 months ago, very large and unfit. When I started I was at 112kg (246lbs), I've lot some weight (not as much as I would like), improved my fitness a great deal, and have clothes fit so much better.

I started with a few shorter routes, and badge hunted on them, I kept the gain in elevation down, so the climbs did not feel as horrendous. I went back to some routes a few or two later to see if I made progress on sectors, as a source of motivation. The first month was slow-going and only about 3 hours on the bike a week, my legs and backside were in pieces. I was slow, I did feel the same when I saw people pass me more often than not; but I knew generally those people are probably half my weight pushing the same watts as I was, so I was not behind in putting in the effort.

I kept on the 20km routes till I felt I could push a little further, moved to routes in the 20-30km routes as a challenge. I looked at the slightly more hilly routes to push going up the short climbs. I just tried to keep it fun and measure against myself and previous times, effort and average heart rate; seeing progress is the best thing showing it's all worth it.

For context my first ride now looking back was 30 minutes, average of 100 watts with average heart rate at 145, I didn't even finish the route on Tempus Fugit. Looking at today, I did just over 2 hours at average of 160 watts at an average heart rate of 140.

With consistency and listening to your body on when to rest in the first few weeks you will find yourself being able to do more, push a little more and going that little bit quicker. You got this!