Jamie:

I cannot remember what happened that has your legs feeling bad. I haven't taken three weeks completely off, but I probably cut back in my taper too much the last three weeks of every race I have ever run. My brother is a prolific "rester". And that has him 0 for 4 or 5 at 100 miles. However, he seems to be able to do the 100K distance pretty good on training that I consider far less than adequate, and he has some respectable times, not fast but not last either. So, I think it is doable to get a little bit of running/biking or whatever and still complete the 100K. I am slated to pace my brother after 100K next weekend to try to get him a finish at 100M. We will see. He is definitely well rested.