Pete,
I really don't think you have enough rides to make any conclusions. The reason is that there is way too much variation in your rides. If I'm calculating correctly, in your 2 downhill rides you see a variation of 10counts/km, and you see the same variation among your 4 flat rides. With such a small sample it is not possible to conclude that the difference between the two groups is 5 times smaller than the variation you see within the groups!