The runs proceeed along the Embarcadero, southward, towards AT&T Park (the San Francisco Giants home turf). The 5 km route turns around less than half-way to the stadium, the 10 km route turns within sight of the stadium, while the 10-miler continues past the stadium, crosses the bridge on 3rd, continues to hug the water on Terry Francois, then goes left, up a hill, on Illinois. Then it's back the way you came. So in addition to being longer, the 10-miler includes a hill. I figured it would be interesting to compare paces at the different distances.
Here's the result, plotted semi-logarithmically. On the x-axis, the placing normalized from 0 to 1. On the y-axis, the log of the per-mile time (which runners call "pace" for some reason even though pace should be the reciprocal of this value: on a log scale, though, one is just the inverse of the other). Results are shown for the 10-miler this year, that 10-km run from late 2008, and the 5 km run held in conjunction with the 10-miler. I did the former two, so for these, my result is indicated with a red cross.
To my surprise, runners in the 10-miler were actually slightly faster than they had been for the 10-km. The 50-percentile is fairly close, but the top half in the 10-miler were clearly faster. There was a clump of 10-milers coming in 11-12 minutes per mile, a clump which is missing from the 10-km run, which was closer to log-normally distributed. That's sort of curious, actually, which suggests a group stuck together to enjoy each other's company.
My pace did what I'd expect: it got slower, significantly slower. I try not to let this bug me: I'd done no hard running on the road yet this year, since my left leg has been bugging me (probably ITB) and I'd done no fast running on the road since early 2009. My goal had been sub-7-minutes, but then goal creep sets in.... Anyway, that aside, with the faster pace among the top half of the population on this run, this resulted in a considerable reduction in my placing.
But then look at the 5 km results. There's a huge spread in 5 km pace. The fastest 5 km guys were up there close to the same pace as the top guys in both the 10-miler this year and the 10-km race from 2008. But from this fast head of the pack, there's a huge range of times, much broader than for either of the other two events. Clearly the 5 km race tended to attract runners who were relatively slower, maybe correlated with less endurance or an unwillingness to be on the road for the longer time required running the much longer distance at a slow pace.
So there's a clear self-selection bias here. You can't compare placings on the 10-miler to those on the 5 km race. Had the 5 km race not available, some, but clearly not all, of the 5 km guys would have done the 10-miler instead. But still there'd be a self-selection bias. Nobody forces anyone to do a DSERC race on a given weekend. If the race isn't what a particular runner is after, he does something else.
An interesting question is how this extrapolates out to the marathon distance. Well, clearly the trend wouldn't continue, as my 10-mile pace (6.95 minutes/mile) would put me at 3:02:40 for a marathon. That scores a lot better than 19.3-percentile. But then the marathon tends to be a "goal" event for people. For many marathoners, it's the only race they do in a given year. The goal is to finish, not finish fast. So the selection bias is different. And it's clear from these plots I would not do a 3:03 marathon right now. My pace would be slower at the 162.2% longer distance.
Anyway, my goal is still a sub-40 10-km. For that, I need to go 8.2% faster over 62% of the distance. Can I manage that? I think so. I just need to get my legs accustomed to running at faster than my usual 8-minute-per-mile pace.