High heart rate after months of Z1/Z2?

I’m M/39 training for general fitness, and specifically to get my mile time back down to 5:00. Right now I’m probably around 5:30.

I’ve spent the last 4-5 months building an aerobic base. Peaking around 35 miles per week, with long runs of 10-15 miles.

I recently took a week or two light/off and am now transitioning into a more specific phase, trying to get back into hard track work. Yesterday when I was trying to run 175m repeats at around 85% effort (RPE ~7-8), my heart rate was shockingly high — hitting about 200bpm each rep. When I sleep it’s typically below 50bpm, closer to 40 when I’m in good shape, and my AET is somewhere around 140bpm.

It was pretty unpleasant and I definitely didn’t feel strong. Is this a common occurrence after a long base-building phase? I’d chalk it up to being just a weird off-day for my body, but even max efforts haven’t gone above 190bpm for me in a very long time.