The problem is, doing more on some days to make up for others when you did nothing is well known not to be as effective as consistent daily progress : http://jonkarpman.blogspot.co.uk/
If I walk 20 miles and my ultimate goal is to get to 10,000 miles, then 20 miles of effort translates into 20 miles of distance covered. It's a linear one-to-one no frills relationship. The units don't even change.
But if I'm doing push ups, my goal of 10,000 is actually a proxy goal for things like upper body strength and physical well being. In that context, the biological impact of 20 push ups x 3 days ≠ 60 push ups x 1 day, and no push ups the next two days.