I have heard people say that the longer endurance races allow saving but can someone confirm this? I assume the longer ones, like 9h Tsukuba and the two 24h ones, will have this option?
Nope, no save. Nor is there to switch between A and B -spec mid-race.
Some endurance events too long for you to do? Do what I've done in past GT games, and just not do them. The world won't end if you don't 100%-complete GT5, believe me.