If you're just riding for 'fitness', forget about the HRM. If you still want to track your miles because it's fun, just get a cheap bike computer, non cadence needed. Just ride hard when you feel like doing it, and enjoy the rest. The metrics are really only important when you want to know exactly how much you're improving and what kind of stimuli you need to keep improving, which is mainly for racers. It's overkill for fitness cyclists unless you specifically enjoy this sort of stuff.

I have maybe a dozen rides around my house of various distances that I mapped a long time ago on my laptop. So, knowing about how long each is, I look at the clock on the microwave on the way out the door, and look at it again on the way in. Minimalist training I guess. I think it more fun when I keep it simple.

For once, I totally agree with you. I used bike computers for years, but haven't for 11 years. My main objective is to have fun which includes trying to keep up on fast group rides.
My improvement or decline is measured by if I can keep up or not and by how I feel after a long day of chasing or climbing.