What would you do for the last “fast” session before the marathon—threshold stuff or V02? Or does it not really matter?
I think it matters. But as you've probably learned, it takes about 10 days to absorb the training effect from a workout. So I'd be looking to do more 'sharpening type stuff. More neuromuscular. Lots of 400s @ MP maybe? Trying to 'hardwire' that pace. 200s even as the date gets even closer. But if you're talking last hard session before a serious taper, I'd opt for VO2 work. Less total volume than previous VO2 sessions but with just as much pace (or slightly faster since you're cutting back the volume). So if you were doing something like 6 X 1K, drop it to 5 X 800 but do them faster.
But as you've probably learned, it takes about 10 days to absorb the training effect from a workout.
Sure. 10 days for any workout, no matter what kind of workout and how hard. It's just a trigger that switches on in about 10 days: not absorbed... not absorbed... BOOM! absorbed!
Or maybe, just maybe, there's a lot of different adaptations that gradually affect different functions (all at the different times).
Actually, a lot of different things could work 10 days out from marathon. I would think that doing long fast intervals (10k pace and faster) would be counterproductive. At this point you should be somewhat fat-adapted, having learned to burn higher fat percentage at around marathon pace. No point losing that adaptation by doing vo2 max 1ks.
So, race pace work and slightly faster (HMP) is good.
Also, shorter intervals to maintain speed reserve/efficiency (maybe 400s at 5k pace with long rest).
Mostly just try to maintain what you already should have, get the feel for the race pace and try to be fresh and comfortable at it.