Doing moderate rides around here, I now also use a powermeter. I think it'd kinda fun trying to keep it in a narrow range, much harder than I would have thought. You know, anything to hold one's attention while trying to ride moderate. It's also interesting to note one's HR response over time. The idea is that, holding the same power, HR will also stay about the same over a couple hours. It if does, you're in great shape and have achieved your aim of aerobic fitness.
Around here, one can't ride far without hills. I can't keep my power that low on hills - I'd get bored stupid - so I let it go up quite a bit, but put a hard limit on it so I don't wind up breathing hard. The thing about "ignore power and listen to your body" is that stress trails power by a lot, going up and going down. It's better to be proactive IME.
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Results matter