I did CBR-SYD-AUH-LHR, spent 21hr with meetings all day, then LHR-MUC, spent 45 hours at a conference with a pres, then MUC-CPH-BEY where I spent 40 odd hours before heading back home.
All ULH flights were in F, J otherwise. I remember every bit of it.
I also did a RTW within 7 days, with 8 flights and 3 stops, but that was more tiring than I had expected.
Perhaps what helps best is few nights of good sleep prior to the trip. Then make sure you rest wherever possible and avoid alcohol/heavy meals.