I ran two 50-milers in three weeks. The first I did in a sluggish 11:55. I wore the wrong shoes and tore up my feet. Not only horrible blisters but a lost toenail (gross!), completely destroyed my heels and irritated my achilles. Not good. I'd never had such problems running a 50 before.
The second 50 I ran in 10:40, much better (though still slow). My achilles bugged me from about mile 15 on so I stayed slow and modified my gate to protect it. Coming in under 11 hours qualified me for Western States 100 so my slow, guarded gate forced me into a lot of clock-watching. It was also pretty hot (around 90 degrees) and only one aid station had ice so my brain boiled most of the day.
6 weeks rest means no running, more swimming and a bit of time at the gym.