Running is a mind game and motivation can come in many forms. For me, I like to read running quotes from Running Greats. Steve Prefontaine is a great source for quotes. For example:
A lot of people run a race to see who is fastest. I run to see who has the most guts, who can punish himself into exhausting pace, and then at the end, punish himself even more. Nobody is going to win a 5,000 meter race after running an easy 2 miles. Not with me. If I loose forcing the pace all the way, well, at least I can live with myself.
This is the kind of thing that i like to read when I feel like running has become monotonous. It doesn't happen all the time, but when it does I usually try and Google some Prefontaine quotes.
To give anything less than your best is to sacrifice the gift.