Many runners don't take the advantage of rest and recovery between races and hard workouts. As a result, what is supposed to be a key workout sometimes turns into a sub-par effort. Not only that, runners that make a habit of abusing recovery time between workouts are subject to an increased risk of illness, injury, and progressively poor running performance.
In it's most basic form, any marathon training program involves repeated tearing down and building up of muscle tissue. Over time, the muscles adapt by increasing the number and size of mitochondria (responsible for energy metabolism), along with enhanced fat burning enzyme activity. Some fast twitch muscle fibers actually take on the characteristics of slow twitch fibers. There is also a higher capillary density, improving blood flow. All of these changes contribute to an improved ability to process oxygen, which translates to faster running with less effort.
The trick is timing and balancing rest periods between hard workouts. Long runs over 2 hours for example, take longer to come back from than a hard interval workout. Most runners should consider a 2 day recovery following any hard workouts. This 1999 study out of Finland measured complete muscle recovery 3 days following an interval workout.
The typical workout schedule might call for a long run on Sunday, intervals on Wednesday, and tempo run or short race on Saturday. Your routine may dictate something different, so don't be afraid to experiment. Recently, I've been having modest success repeating this 3 day cycle:
DAY 1: very easy 30-40 minutes
DAY 2: intervals
DAY 3: long run easy- 2 hours
DAY 4: very easy 30-40 minutes
DAY 5: intervals or tempo
DAY 6: long run easy- 2 hours
DAY 7: very easy 30-40 minutes
I've found that I can do the long easy run comfortably the day following a 25-30 minute interval session. This schedule allows me to get 2 decent long runs and 2 intense workouts, and 3 easy days every week.
Whatever you decide, never underestimate the importance of good rest before and after hard workouts.