It depends what you're aiming for really. If you just want "get you round" fitness, then you can get away with a lot less than a 2/3rds distance training run. I did my WHW run (95 miles) in 2005 with a 27 miler as my longest run and before Christmas with a 38 miler.

I know an ultra runner who never trains longer than 3 hrs working on the basis that recovery time starts eating into training time if your long runs are too long.

I would have prefered to have been able to clock up higher weekly mileages than my peak of 60 miles because I sure I would have slowed less if I had.

I think if you're in the 70-100 range you shouldn't have any problem. Mentally, I'd find it hard to reach the end of a 42 miler and then turn round and run all the way back, I prefer A to B where every step is progressing you towards the finish. At least you'll know exactly where the halfway point is though.

Any particular reason for this run or just a personal challenge?