I just thought of something else.
Most of the time I read this board I get the feeling that many people might be over concerned with, training schedules, time, food, rest, goals, PR's…blah blah. After a while everything gets old. While I agree, you need to have goals, why not change the goal?
Try just going out for a run. Sign up for a marathon with a friend and turn it into a vacation. Don't pile up the miles so you can put your head down and go for a PR. Run slower than normal…look around, talk to people, talk to your friend, get your medal, have a post race beer, have 6 post race beers.
Maybe changing the idea of what your goals can be will refresh things for you?
Just a thought from a guy who doesn't think about PR's very often but likes to run marathons.