Fixed schedule is one possible system that will push you to keep going even while you go through demotivated period. Same with "other people's expectations". Seeing other people exercise is motivating for a lot of people. And being seen is one more nudge to actually do it today. Moreover, many people are motivated by competition. That does not need to be official competition or large scale one. Pretty often it is just "I want to keep doing it as long as him" or "I think I could get bigger score then mx123Throwaway". For many people, the social aspect is what turns activity from "boring chore I do because I think I have to" to "I actually like doing it".

Underappreciated aspect of long term exercising is "how to keep doing it long term" and "how to prevent it from becoming yet another annoying dot on my todo list".