Typically 20-30 miles a week here. I did my first trail marathon off a longest of 15, and first ultra off a longest of 21. They do get easier as you do more and like Cougie above I wouldn't necessarily recommend it as a strategy, but if you're reasonably pig headed you can normally get round.
Don't know if you found a place - give us a wave at checkpoint 2 if you did and help yourself to cake! I've done it a couple of time and reckon it's the perfect next step from the marathon. Not too far, well marked, well supported and pie at the end