From “11 ways to make running fun” at Running Nut:
Running constantly alternates between good and bad, easy and hard, exhilarating and boring. The average runner experiences long periods of drudgery, interrupted by a brief period of exhilaration. By keeping running fun you can eliminate these long periods of drudgery. You can also increase the duration and intensity of these periods of exhilaration. Perhaps, you can even make running so fun that it all blends into a constant period of exhilaration.
A good article that lists 11 things to do that’ll help you maintain the fun in running. My favourite, of course is number 5:
5. Run With Music
A recommended read.