Here's my Scientific Wild-Ass Guess (SWAG) for a fair score of fitness. It seems to make sense for me and might for others as well.

     There's a simple model out there that says two race performances over different distances are equivalant if pace gets five percent slower for each doubling of distance. That means 5 Km in 20:00 is equivalent to 10 Km in 42:00, five percent slower, 2.1 times more time for twice the distance. I have a run-times-predictor web page using this formula. It has worked well for me. It works out to time being distance to the 1.07 power.

     The trouble with "equivalent" times is they assume we're already in good-enough shape to run both distances. While 5 Km in 30:00 and 10 Km in 63:00 may be equivalent race performances, I've been in just good enough shape to limp through three miles, so I could run 5 Km in 30:00, but I couldn't do 10 Km in 63:00 simply because it was too far for me at the time. A couple of years ago, I was still able to run a half-marathon race in 2:03:30, the formula says that's the same as a 4:19:21 marathon, but there was no way I was able to do the latter simply because I wasn't in shape to do the distance. So we need a factor that says I'm in better shape just because I'm racing further.

     So I added another five percent for every factor of two in distance. That works out to time being distance to the 1.14 power.

     For my reference point, I decided that a half marathon in an hour and a half (90 minutes) would be a good effort to be 100 points.

     If you use my equivalent-running-times web page and mouse over any of the distances and times, then you'll get a pop-up text (a "title" in HTML jargon) with this fitness score.