"Many say 20 miles is sufficient. Others, like Mr. Galloway, recommend conquering at least the full marathon distance in training. Still, whatever the distance of the longest long run, novices can?t go from zero to 26 miles overnight, which is why most plans are at least 12 weeks long, and some last up to 30 weeks. What?s more, most coaches and exercise physiologists recommend against even starting a marathon program until you have regularly run shorter distances for a couple of years. "
IMO, the difference between the elite athlete doing 20miler long runs and others is that at the same distance, the elite athlete is not struggling to complete the run, and they have more than one speed at which to complete the 20-miler, slow medium fast... so by the time they do the 26 miles, it's just a matter or how fast will they RACE it in.