I did something I normally don't do on my bike rides today.
I rode for my heart rate.
I obsessed about keeping it around 75% of my predicted max. It was rather slow going. I got passed by an old guy on a really old Schwinn road bike.
I did get the HR up sprinting for a light. But other than that.....slow and steady, with one eye on the road, and the other eye on my HR display.