I used to try and think of exercise sessions just as something routine I did in the same way as I would brush my teeth. Not a big deal or effort as such. I would have a rough plan for the week but if on the day i had, say, a swim planned, I struggled to get going then i would gave myself a choice of swim, run, gym session, class...something would be the least horrible option
. Its good to mix up what you do anyway, both from a fitness and enjoyment/motivational point of view. Eventually it did become a habit to do something almost every day, even if its just half an hour, it is now a completely routine thing for me to do, it is rare that it feels like a big effort to train.
You know that you always, always feel better after exercise, the only exception to this is if you are ill. Think of that, then stop contemplating how hard the prospect seems, switch your brain off and get on with it, pull your trainers on
even once in gym kit it is harder to not do it!
Mercury's approach of starting a session, giving yourself permission to stop after 20 mins is also a good one that I have used. It's very rare that you will actually want to stop then.
Finally, a big thing for me is a year or two ago I found a sport (kickboxing) that I love and I am relishing getting better at. I still lift weights and train at the gym 2-3 times a week, sometimes run or swim if I feel like it or am away from home, but that general fitness is in my mind helping to improve my kickboxing.