On my fifth day, I find that they are getting harder to do, not easier. I think that is the necessary stress. My first set of 15 is difficult, second set of 15 easier, and the remaining sets of ten become more challenging. I am only up to 80/day, will get to 100 on Monday. My sets are spread out over hours.
If over age 20, I wonder whether it might make more sense to put in a day of push-up rest every few days for recovery. My goal of course is 100/day, but more specifically 50 in a row. I don't need to do 100 in a row.
What are you finding?