The gym was never for me, I could never find that drive to challenge myself and push myself further, but for me the issue was always people being around when I was working out, I just really didn't like that. I know no one gave a shit about what I was doing in the gym, but I always felt ridiculously pressured about looking like a twat. Stupid but just the way it is for me. Anyway, I used to run of a night when no one was around, I'd set up a shit ton of different playlists and just whack one of those on, there was always something really quite relaxing about running around in the dark with no one about and awesome music going, was especially good of a winter because you're either running to stay warm or the rain will cool you down.
My drive when I did run (I should really get back into it) came from signing up to a 10k, not being able to run whatsoever, and just practicing. Music made it so much more enjoyable, and not being boiling hot as your were outside helped a lot. I also signed up to Tough Mudder in September and that forced me into the gym because fuck running 12 miles without any training. I hated every moment in the gym, but it got me where I needed to be. I guess I just wanted to know I'd be able to finish the run / obstacle course, even though there was some obstacles I couldn't do in Tough Mudder because I was so broken, I still got around all 12 miles of it, and there's so much satisfaction at the end of that, all that work you put in really does feel worth it. So yeah, charity runs and shit like that can be a good way to motivate yourself.
I know all of the above may be a little specific to me, but thought I'd share what motivated me anyway.