Head for the pits, the "save" option shows up in the pop up menu when you enter the pits. Before Spec II you just had to define what tyres and how much fuel. Now, there's another field, for the save feature.
If you do save, you get a message telling you to quit the game. You can't quit only THE RACE after saving. The saving is about the entire game, you will freeze it. And whenever you want to play it again you will have no option but to: a) continue the race; b) quit the race.
So, setup a second PSN ID to play online with your friends while your main account is blocked by an ongoing long enduro.