I really like this quote by George Sheehan. It emphasizes the point that so much of the battle that rages in our quest to "make it fit" occurs in our own heads...
"It's very hard in the beginning to understand that the whole idea is not to beat the other runners. Eventually you learn that the competition is against the little voice inside you that wants to quit."
One of my favorite ways to combat this "quitting attitude" that my mind sometimes gives my body is by chanting a certain catch-phrase in my head over and over again. It gives my mind something else to think about rather than concentrating on wanting to "hand-in-the-towel" during any given workout. Any motivating phrase can work, as long as it's short and personal to you. Some that I have used recently are...
"Move that body."
Find something that works for you and try saying it to yourself when the going gets tough during your workouts. This definitely helps me to win the race that regularly occurs between my body and my mind!