Been there done that, went for ~400 lbs down to ~200 lbs. Diet and exercise which included biking.

Let's face it, we got fat for a reason, we are lazy and eat too much. You have very poor muscle mass. You need to build up that muscle. You build that muscle by exercising. At first it hurts and is very tiring. Keep pushing yourself a little more each time and it gets easier.

If you are extremely sore and tired the next day then maybe, just maybe you should take the day off. If you have enough energy to get up and go to work, then you should have enough energy to ride again.

Ride further and further. Don't worry about 'fast' yet - if ever. I was getting all wrapped up in performance and realized I hated it. I ride because I like too now, I look at stuff, stop when I want to and get done when I get done.

I started riding after a break of over 15 years about 8 years ago. Started slow and kept going further and longer. I ride for 20-30 miles without much issue now. My absolutely favorite ride is about 60 miles. A wonderful out and back on the lower half of the Razorback Greenway in NW Arkansas. I extend it via the Cato Springs trail all the way out to Kessler mountain regional park. Sometimes I poke along, sometimes I ride fast it depends on my feelings at the time and the weather and how much I am enjoying the scenery. The point it, I love it, so I stick with it. If you just can't do riding and feel badly and never enjoy it perhaps biking isn't for you. Try hiking, or swimming, or ???. Just keep at it.

Wish you luck friend!