Some relax their fits. Others find they don't need to. Others do lots of strengthening exercises so they can hold aggressive positions for long rides. Others decide they want to target the long events, so they set their bike up in a position to suit that and ride like that all the time, to give them the benefit of training in position. That will put them in a sub-optimal position for short events, but you have to decide what your priorities are and train for those.
I believe Hippy rides his TT bike in the same position for everything up to 24 hours, and it doesn't seem to slow him down at any distance. I believe he's spent a fair bit of time and money on getting properly fitted to iron out any physiological niggles though? I believe he's doing the Transcon, but I don't know what he's doing about that.