Back in July or August 2006, I did MHT-CLT-BHM-CLT-MHT, followed by BOS-LAX-BNE-SYD-SFO-BOS, and finally MHT-IAD-EZE-MVD-EZE-IAD-MHT. The first two were for work, while the 3rd was for vacation. Not sure how many miles, but my backside was pretty sore after that many miles in Y, including a middle-middle on the 744 from LAX-BNE.