I think it really depends on your base fitness... how many marathons have you run in the past? 2 or 30? When was your last bigger training block? 6 months ago or 3 years ago?
I have done three races like that. Silver Rush 50 miler in leadviile. A buddy came out to run it and convinced me 5 days before it to run it with him... I hadn't been training for about 6 months, and did one 7 mile run the day he convinced me to sign up. Was on pace for a 9ish hour finish until about mile 30 then limped in at about the 11:30 mark. So it didn't go terribly well.
Devil on the Divide 50k. This one went really well. Had about a month to 'train' and got in a couple of 20+ milers and ran at least 6 miles every day. Toed the line not in the best shape but fresh and finished 13th in a fairly competitive field. Was in the top 10 until the last mile but had no kick and nothing left to hold on.
Estes Epic 50k. Got a free entry the day before and hadn't trained at all (like 15-20 miles per week for 3 months) but had trained and finished the Bighorn 100 in June (Estes Epic was in September) so had a good base of endurance to pull from. Finished in 6th place.
But I have a pretty solid endurance foundation and can tolerate an abnormal amount of discomfort and still be able to push and enjoy it. I have friends who can run 100 milers on any given day regardless of training, and I know folks who have to trail for a couple a few months to get through a half marathon. So it also depends on your own body and how it naturally handles this stuff.
SO it can be done, but experience and base fitness plays a pretty big role. Just assess and go in with realistic expectations and you'll have a great run. Good luck!
Class of '00
Gig 'em!