Your muscles are probably getting plugged up with waste products. The short easy spin helps clear them. That will keep getting better as your aerobic fitness goes up, which will bring the quantity of waste products down. Try to simply do longer rides. When I started riding again at 50, I'd ride away from home until I got tired, then ride back. It wasn't that long before I was riding 60 miles, mostly flat.

I think the thing to do at this stage is simply hours on the bike per week, as many as you have time for. Go at a pace that you can keep up. Don't push it, just ride for the miles. If that's what you're already doing, cool. Then start riding hills. Hills are where it's at for cycling.

When I quit smoking I took a vow that I couldn't do my most favoritest thing if I smoked even one cigarette. After a few not-being-able-to-do-that-things, I quit. The physical craving goes away in 3 days, so it's not really as bad as it might be. Mental craving is another story: that's what gets the best of folks. You have to have a good reason not to smoke IME.