My longest was LHR-JFK-LHR-DXB-LHR-NRT-HKG-MEL - only the first in F the rest in J. I can't say I was particularly chipper by the end, but I wasn't dead
Make sure you enjoy the amenities which may be on offer (eg massages, cabanas in HKG, etc..) and walk as much as possible when you can to get the blood moving.