In reply to JLS:
I'll throw my hat in:
My 10km personal best is 32 min (long time ago
) and would have been able to do around 8-10 1km reps at wr pace (2:55 /km). If i would break it into 400m intervals, i suppose i could got to 20km (which would take more than 15 hours), but there is no way i could have gone past 30km, no matter how short the intervals, because you are just spent after doing this for a couple hours.
So i think you need be able to do it in 1km or longer intervals. To do 42 1km intervals at that pace, i am pretty sure that you need to be a sub 30 min@10km runner, which translates into a sub 2:20 marathon.