Another way to think about this is that if you calculate the mileage from midnight, where he was roughly on top of Mt. Lincoln, it makes for a longer day than 21 miles and really doesn't seem that bad through that section.
How many miles is it from Mt. Lincoln to Galehead Hut?
Shouldn't it be calculated that way anyway? Not based on when he stops to rest but on a calendar day that starts and ends at midnight? The previous day's 48 would be reduced also.