Congrats on getting back in the saddle, I'm 67 and my Mtn bike has taken over my bike duties for the same purpose as you. Like you, I had been off the bike for several years.
The roads here (So. Flo) are full of drunks, retirees (I'm one) daydreamers and texters and the fact is bicyclists get swatted more often than other places. So I ride almost exclusively on our generously provided sidewalks, but with their many obstructions necessitating riding through gravel, grass, and rocks, the best I can squeeze out of it is 17 mph and I do my regular daily route of 12-14 miles in ~ an hour.
My resting heart rate is down and the weight is off by 14 pounds with a target of 10 pounds to go.
North coast Joe has a GR8 idea there! I wish I had the space.