That's so hard to answer Aaron.
Elevation, terrain, all the other variables…Soak your feet on day two, don't treat them right, and blisters change everything immediately.
I did a 5:35 marathon yesterday morning (actually it was only 24.5 miles, but who's counting!)…90% on singletrack, lots of elevation loss and gain, lots of fairly rough terrain (rocky stream beds without much trail)- I walked the steeper uphills.
I felt about as good (or bad) at the end of it as doing 50K in the Sierra (including Forester Pass and Mt. Whitney) with an ~18 pound pack in about 16 hours.
But this is me.
I don't think you can undervalue the mental aspect that goes into logging big miles.
Training books/coaches will give all sorts of mileage formulas- I'm sure as a marathoner you're familiar- but I think you can easily toss all that out the window.
Prior to my marathon yesterday, I hadn't run over 12 miles in a month and a half. By any book or chart, I shouldn't have been prepared.
But mentally, I felt good and confident, so it went fine and I finished.
I have a friend that's an ultrarunner (100+ mile races) that inspired me to just go out and run big miles and forget the formulas.
I told him I was hoping to go out and do a marathon soon but was working my mileage up.
His response:
"Just go do it. Find a route that get's you 13.1 miles from home with no escapes. Then you have to do it. It's all mental. And if you trash yourself from being a bit underprepared, you'll be that much mentally stronger next time."
I think he's totally right. Who can ever really be prepared for 100+ miles?