It looks like you're new here. If you want to get involved, click one of these buttons!
Can't find a source right now but I think it's generally in the region of 80-85% for a well trained athlete. I had a quick look at my Noakes book and he refers to Jack Daniels' VDOT training and says that threshold is equivalent to around 88% VO2max so marathon pace will be a bit lower.
Obviously no one will know their actual VO2max unless they've done a lab test. You can get an estimate from various sites from putting in a race distance, e.g. 5k which is run close to VO2max although 3k is closer, so I suppose you could estimate it from there. Jack Daniels is your man though if you want to look it up.
I've been looking for research and haven't actually come across a lot. Although I suspect that's my search strategy rather than a lack of data.
This might be worth a read, although I don't know who he is.
I would've had a look at my Pfitzinger and Douglas book but I think I might have leant it to someone. They definitely single out VO2max as one of the key variables worth training for marathon performance - although it's kind of ''icing on the cake'' after things like aerobic endurance and lactate threshold - so I'm sure they've got a bit to say about it. Might be worth searching on them as well.