It can be hard to take a rest because things have been going well. The key is to take the rest before your body breaks down so far it takes you out of the game for a long time. The old adage "an ounce of prevention is better than a pound of cure" is completely right. I've struggled with several injuries and I have learned that it takes so little effort to prevent something but getting healthy is very hard.
I read somewhere recently that some coach only coached people that he had to hold back. I think that concept can be applied to many situations. Everybody does something a little more than is healthy. Wether that is playing a video game eight hours a day or studying 15 hours a day it is often good to take a rest or at least hold yourself back.
I often tell people to stay motivated. Burnout, overtraining, stress, and not taking a rest every now and then contribute to loss of motivation. Once your motivation is gone it can be hard to get back. I think this is one of the advantages of extracurriculars (like running) in school. It's not that it actually makes you smarter it's that it makes you focus on something else so you aren't thinking about school. From September until May almost every Saturday is taken up with our cross country or track teams traveling somewhere to race. The event usually takes up most of the day and most people don't even touch their homework on Saturday. I think this short little weekly break is often enough to reenergize people to hit the books during the week. Another little diversion is the daily practice and for two or three hours each day people more or less accomplish nothing as far as academics are concerned. This convenient break keeps people motivated to spend four or five hours every night working on homework and projects instead of a strait 14 hour grind.
At Philmont (a camp I worked at for two summers) the Conservation department (which I was part of both years) had the moto "work hard, play hard". So simple and so true. When you do something, do it. When you rest, rest hard.