Don't admit to that ! I thought you were being really astute as a newbie equestrian photographer ! Seriously though, I have found over the years that you need a little luck to get on.
Oops! Actually in my case, it wasn't important. My client wasn't too interested in who was in the shots, so I wasn't worried about getting the winners. For my own benefit, it would have been nice, and next time, I'll check the ride order sheets to make sure I'm in the right place at the right time. It's actually pretty easy to do and when I was asked to get shots of two of the riders, I just set up at an obstacle and waited for them.
Also, I really had a great time and considering I'm near one of the prime US equestrian locations (Ocala, Florida), I think I might get into shooting more Eventing. The people were really friendly and the photographers that were covering the event mostly shoot to sell souvenir photos to the riders, so there's definitely a place for other niches.