BlazingPedals

Work your way up to 25-30 mile rides on week evenings, and once you get there gradually increase the intensity. 500 miles/week not required. In fact, for aerobic conditioning, about an hour of high-intensity every other day, with shorter easier recovery rides in between, is optimal. Training with a heart rate monitor is sort of out-of-fashion these days; now everyone uses power meters. I like HRMs because without them most people will overestimate how hard they're working and not train hard enough. Then they wonder why they're not getting results. It's important to note that you don't get stronger while you're training hard; you get stronger while you're resting. So train hard and then rest harder.

The models that use a chest strap aren't very expensive; I wouldn't bother with trying to match one with a smartphone, I'd just get a package.

bblair

If your goal is simply to improve your stamina, get faster, maybe lose some weight or get more fit....just ride. That's it. HR monitor not even necessary unless you like to crunch the data. You'll know when you are improving; you just feel it.

Just ride. It's not that complicated.

pdlamb

At 44, unless your doctor says you need to monitor your heart rate, you probably don't. As other (sensible) posters have noted, the way to develop stamina is to ride frequently, gradually increasing the length of your rides. Ride what you've got, as far as you can. That's the key.

I re-started cycling about your age, riding with my children. First it was around the neighborhood, then it was down to the bookstore (5 miles away) for coffee; my wife usually drove the car down and hauled us home. By the end of the summer, we'd usually do round trips every week. I kept at it, and 10 years later my daughter and I rode across the country. Four years after that, I got my first heart rate monitor. The HRM feeds the nerd in me, but honestly, it doesn't do much for my conditioning.