Exactly JeremyG. Most don't even know people do this kind of madness. "All in one go?" is the usual response. I'm not quick either (I was stunned and delighted to get under 1:40 for a half marathon for the first time this year) but it seems a have a bit of mental strength to keep going.
I'm doing this. 12 months ago, I was an ultra novice. I did the Malvern Hills ultra running 55 miles in May and loved it. Now I'm signed up for the SDW100 (holy sh!t) and I'm doing the SDW50 as training. I can't quite get my head around it.
If you had told me to run for another 45 miles as I staggered across the finish line of the Malvern Hills, I would have laughed and then tried to kill you.