Got quite far while ‘revising.’ I’ve done all except the 9+ hour ones on A-spec. Nurburgring 4 hours is ok if you have the Red Bull. You can just do 2 hours 30 then leave the race till the end and come back still in the lead. Indy’s a little dull, but because the laps go quickly its very easy to get a bit lost in chasing the best laptime or fastest speed while slipstreaming. Done them all on b-spec.
If you do them in stints and have a break to watch TV or read a book ( or maybe even stretch to going to a lecture) then it’s a lot more manageable. Can’t face 9 hours of wherever it is though.
Edit @David: Yeah suzuka is a bit much, but at least it’s a challenging circuit to keep you interested. Tsukuba will be left unless they introduce pausing endurance races so you can pick up another day.