So it turns out I was cheating myself. I fixed that tonight.
Tonight, I made sure to drop down closer to the ground in my push ups. As a result, I was shaking terribly by the time I got to 20. So I did my running in place for five minutes, and dropped my sets to five of 20 instead.
Based on the way my arms feel tonight, I’m sure I was cheating myself out of a proper workout. I’m glad I’m back on track.