Never in GT-x have you been able to save during a race. Between races in a series, yes. But not while in a race. So it shouldn't be a surprise that the 24 hrs (and 9 hrs and 8 hrs etc.) have to be done straight through without saving. But that doesn't mean not pausing. I had my PS2 on for 5 days straight when I did it in a-spec. No problems whatever.