First off, this is a beautiful photograph. I think you've pretty much nailed it with the composition and lighting. You have a really strong focus point, despite the fact that the riders are facing away. They're bright and well exposed, and the little bit of colour draws the eye instantly.
The way you've framed the shot with the narrow passage also adds more interest to the riders, and is a great lead in to the shot. The first thing that struck me when I saw this shot was its depth.
Overall, you've executed this shot excellently. The only thing I feel that's lacking is the impact. It IS a great shot, but it only draws my (great) interest, it doesn't evoke any strong emotional response from me. Depending on what you seek in photography, this may or may not be relevant.
Hope this helps Congratulations with your achievement, you deserved it! I imagine this shot would be stunning when blown up and printed.