Fun & Fitness over 50

Specifically, I'm having trouble keeping my heart rate up where I want it; at least at the beginning of a ride. I guess that's a good thing. It means I'm getting fitter.
On my ride this AM, I decided not to worry about my cadence but just about keeping my heart rate in Zone 3, that is, 70-79%MHR.
Usually, if I check it and it's about 130bpm, I don't have to worry about keeping it there for 30 seconds or so but this morning I went from 135 down to 120 in about 5 seconds of slacking off and it wasn't like I stopped pedaling, just not hammering it. I decided I'd better keep my cadence up there in the 90-100 range. That's a better way to ensure that my heart rate stays consistent. Also, when warming up, my HR is sometimes below 80 which was impossible when I started riding. I used to get in the 90s just getting off the couch! I suppose that's progress! :-)