What motivates you to exercise? I think this is an interesting question because there's lots in common with this and motivation in relation to learning. In lots of ways, it's easier to investigate and analyse than the motivation of learning. There's less of the philosophical baggage for a start. It's very difficult to know if somebody else has truly learned something or not, whereas you are in less murky waters when it comes to physical behaviour. As well as that, it's accepted that exercise may be worth doing even if it isn't enjoyable, whereas the waters get muddied much more in that respect with learning.
I've been trying to compile a list of techniques that I've spotted. Here are the ones that I've noticed.
1) Really really care
I guess this is just deciding that exercise is top priority or near top priority in your life. If you can make yourself care enough, then you'll forego other enjoyable things to exercise.
2) Really really care about something which requires you to be fit
If you decide that you really care that, say, you want to be a fantastic tennis player, you'll probaby be motivated enough to exercise to be fit enough to achieve that and forego other things in that aim.
3) Find a way to make exercise enjoyable
People will have personal preferences for some forms of exercise over others. I only discovered relatively recently that I actually really enjoy swimming, something of a surprise to me given that I hated it when I was younger. Other people enjoy being outside. For a lot of people making exercise social is a very important way of making it enjoyable. However, not everybody is extrovert of course, something also often forgotten when discussing social dimensions of learning.
4) Make exercise into a game
It's quite interesting to read the Amazon reviews for the Wii Fit - it really works for some people and not for others. I'm a definite fan myself, even though it's hard a perfect game. This is also the technique used by WeightWatchers and #GameDevDiet. See also the Gamification of Bookification, Chore Wars and SuperBetter.
5) Trick yourself into making it a habit
I guess this is a case of figuring out a way to maximise the chances that exercise could possibly become a habit, probably by fitting it around other habits, and then generating the will power to manage the three weeks you are supposed to keep something up for to generate the momentum for it to become a permanent habit.
6) Find a way to cause yourself pain if you don't exercise
This might be letting down a sports team, having to admit to your friends who sponsored you that you didn't complete what you aimed to, causing yourself financial pain when you see your gym membership wasted or similiar. Looking at the educational parallel of this, I think one of the chief reasons people sign up for formal education courses is this one, and I don't think it should be underestimated. There's an article about such commitment devices in business here that's a quick read.