I was getting myself into a situation in which I wasn't out of breath but felt like I just had to stop. I stop for a few minutes, then ride for 5-10 minutes and feel I had to stop again repetitively. I got an HRM (Wahoo Tickr Fit) and figured it out. I get to feeling exuberant, my HR would go to 145 for 10 minutes, and then I'd have to limp through the rest of my ride. (I just turned 75, and I'm overweight, so 145 probably isn't very far from my max. I can't imagine doing a 30 minute LHTR test.) The HRM allowed me to stay under 140 until my body could handle it.
This is the first season I've used an HRM, and it's been very useful.