Well, let's see, depends on a lot. I have one course where I have run both 50 and 100 miles...9:56 for the 50, 23:02 for the hunnerd. These two races were a couple of years apart and my conditioning was better for the 100 than it was when I did the 50. There is another course where my PR in 100K is 15:28 and I did 100 there in 27:10 (these races were 2 months apart, so conditioning was similar. Some research of race results (where 50/100 mile race options are available) might provide more insight as well. Good luck, Lynn B