In reply to SouthernSteve:
I agree, 3-4 weeks isn't a huge amount of time.
You'll feel worse but if you had a big base it's not that long. You'd lose speed quicker than you lose stamina so for a 50mile race I doubt it will have that much impact to be honest. If it was a 5k race I think you'd be significantly slower. In a 50 there's so much else to manage and go wrong, actual fitness is just one component and with a solid base that's not that long away.