The best way to train a couple of days before a marathon is to stop training altogether.
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 t
Dr. Gabe Mirkin has been a radio talk show host for 25 years and practicing physician for more than 40 years; he is board certified in four specialties, including sports medicine. Read or listen to hundreds of his fitness and health reports at http://www.DrMirkin.com