Athletes will spend months and month building themselves into highly efficient fat burning machines (ideal for the marathon) but then throw their metabolism way off with intense VO2 max type sessions in the last two weeks before the race and then crash and burn around mile 20-22 OK, that probably gives too much credit to most people in suggesting that they actually enter the marathon peaking phase as "efficient fat burning machines", but essentially you don't want to get 'too sharp' when preparing for the marathon, especially as the duration of the race increases beyond 3 hours. VO2 max sessions will have the corresponding effect of increasing the ratio of carbs burned in relation to fats. If you train your body to burn carbs at too high a ratio early in the marathon you'll run out of gas at the 20+ mile mark and then wonder "how many additional gels should I have carried?" and start "Critique my Marathon Nutrition Plan" threads when the problem was in the sequencing of the training. Obviously, if you don't do any sharpening work for the shorter events, you run the risk of never getting out of first gear, so vo2max work does have value as a peaking element for events around 15-20 minutes. For the marathon though, too much vo2max oriented work close to the race (especially if it is accompanied by an excessive decrease in mileage) will make the runner too sharp and set up potential disaster in the last 5k. Yes, Pfitzinger does have what he calls "vo2max" sessions of repeat 600s at 5k pace at the ends of his programs, but unless you are running your 5ks faster than the world record, 5k pace is quite different than vo2max pace for most of the target audience running close to 20 minutes. A relatively short distance (600m) at a less than all-out pace (5k pace) in manageable quantities (7-8 reps) is normally fine for tuning up without changing the body's metabolic calibration as it relates to demands of a 3+ hour race.