Enjoyment is all the motivation you need! I lost my running mojo a while back but the last few years I have only run when I wanted to and only done races I know I will enjoy. Really got me back into it again although only 2 or 3 runs a week and nowhere near the miles I used to do.
Park run is a great way to get back into running. I don't do it now but used it as my rehab after a long lay off through injury. I think I clocked up 103 runs on the end. (I stopped when I realised I was becoming a bit of a 5k road race speed merchant to the detriment of the long hilly off road stuff!)