I run about 30-35 miles a week and have really come to enjoy it. In the beginning it was just to train for a half marathon - but after reading, Born to Run and UltraMarathon Man, I now look forward to it. Its amazing to see what the human body can do. The pain makes me feel alive - I hope that makes sense. Summer heat is brutal for a runner (although I have heard of people that prefer the heat) so I either run early in the am or late evenings.
In the book UltraMarathon Man, the writter Dean Karnazes was relaying a story from his high school days on the track team. He had just won a long distance race by pushing his body to the limits. He coach said , " How did that feel?". He replied " Its felt really good !". The coach replied " Then you did something wrong - its supposed to hurt like hell." I thought that was such a funny quote and yet so true - you are left wondering - can you push more?
Deep down the motivation is perhaps always being able to fit into my favorite pair of jeans :)
Billy has some great advise regarding weights in his post. I personally have never hit the weights hard - but know people who have and his advise works.