This is tricky and complex. Taking time to rest isn't a matter of compromise. You don't compromise anything by seeking the right balance. Movies about heroic warriors perpetuate the idea that the harder you can train without collapsing, the stronger you will be. OK, it's basic physiology that stressing a muscle in a workout tears it down a little and stimulates it to grow back stronger. That's true of all physiological systems and mental skills. But the "growing back" part is too easily neglected. In any exercise, you can reach a point beyond which there aren't enough hours left in the night to fully recover before the next day--so the next day's workout begins with less muscle or resilience than the day before, and the training effect begins to reverse. Symptoms of improper balance betweeen stress and rest include a "stale" or "flat" feeling, a slump in performance, and then--inevitably, sooner or later--illness or injury. And if you don't learn, you could experience burnout and permanent disability. I've seen it happen to a lot of people.
Part of the complexity is that different kinds of exercise require different amounts of recovery. Speed work requires more recovery time between sessions than slow base-building. It may be counterintuitive, but twelve quarter-mile intervals totaling just three fast miles (or six miles overall, if you count the alternating slow laps) may need three times as much rest between sessions as longer but slower 10-milers do. Similarly, intense weight-lifting routines require two or three times as much rest between sessions as situps.
Another complexity is that the physical exercise is not the only source of stress in your life, so it's not the only stress you have to take into consideration in finding optimal rest and regeneration. Ever since the pioneer endocrinologist Hans Selye began his research on the "stresses of life" almost a century ago, we've understood that while such varied experiences as financial trouble, a car crash, the death of a spouse, a new baby, or the planning for a wedding are all very different, their effects on an individual can add up. A runner who is coping with heavy stress at work or home, whether emotional, mental, or physical, may not be able to carry as much workload in his or her training as one who feels relaxed and on top of the world. On the other hand, if you've already built some endurance as a runner, you can probably handle more stress in your life as a whole than you could if you were sendentary. Again, the ideal regimen is to find a balance between the cumulative stress of everything that's happening, including the running, and the amount of rest (sleep, days off, easy runs,) needed to keep building endurance.
--From an Appendex to the book The Longest Run: a Lifelong Runner, an Iconic Ultramarathon,
and the Case for Human Endurance, to be published in October. Copyright 2012, Ed Ayres