I'm finding that I have a surprising amount of willpower for short, miserable mostly-urban winter rides at the moment, because if I want to get this ankle back to a decent level of stamina it's either that or the turbo trainer, and the idea of not being able to do proper bike rides in the summer is a strong motivator.
I've never found fitness in itself to be a source of motivation for anything, as opposed to being able to do some future thing without dying horribly, which has on occasion motivated me to get out on my bike when I normally wouldn't be arsed. Often this decision has taken the form of a spontaneous "is there a good reason *not* to go for a ride?", which fits with the 'just do it' logic.
I find physiotherapy pledges are easier than sweat pledges, as there's a short-term, tangible goal. Boring as hell, though.
And on that note, I should go and ride my bike...