Why do you expect that a rational person would necessarily value the optimum amount of exercise (presumably optimal for health) over everything else they might have done with their time this week?

Most likely because getting regular exercise is a pretty good investment of time. Of course some people might rationally choose not to make the investment for whatever reason, but if someone doesn't exercise regularly there is an excellent chance that it's akrasia at work.

One can ask if rational people are less likely to fall victim to akrasia. My guess is that they are, since a rational person is likely to have a better understanding of how his brain works. So he is in a better position to come up with ways to act consistently with his better judgment.