How long you can exercise a muscle depends on how much sugar you can store in that muscle before you start to exercise. Dr. Dave Costill of Ball State University asked one group of highly-trained runners to jog at a slow pace on the two days prior to measuring their muscle sugars and another group to not jog at all. The runners who didn’t jog stored the most sugar.
Studies on bicycle racers show that their muscles will load maximally with sugar when they take a very hard workout 4 days prior to racing and then ride easily for the next three days. However, Exhaustive running three or four days before a marathon will decrease the amount of sugar that muscles can store, presumably because hard running damages muscles and interferes with their ability to store sugar.
There is no evidence that a single hard workout on the week before a race will help a runner during that race. Hard training tears down muscles and it take several days for the muscles to heal sufficiently to improve performance. So, the best way for a runner to prepare for a marathon is to run at a slow pace on the fifth, fourth and third day before a marathon and not to run at all on the last two days beforehand.