To the o/p:

I wonder if you are on the right bike for YOU.
When I got back into it, I ended up on a bike that was too small.
Quick 6-10 mile runs with it were no big deal, but anything longer, say like 12 miles, was incredibly tiring.
It may be worth it to go to a bike shop that does bike fitting and pay for the service. At the very least, you'll end up with the information on a correct fit for you.

Just a few other things I've found.
It's o.k. to gear down and be slow. I had a heck of a hard time with this at first. I finally got it into my head to find a gear that I could ride in and pedal consistently without gasping for air.

It's also o.k. to take rest breaks. We're not in a race or marathon here. We're getting fit enough on the bike to ride the distance we want to. Keep goals realistic, according to your abilities.

The most important one is hydration. I just started using a bottle cage with a bottle filled with nothing but cold tap water. It changes some rides from a gasping, painful experience into something enjoyable. I still stop for coffees just about every ride. The water drinking thing makes me feel a whole lot better on the bike.

-D.S.