Victoria Pendleton tried horse racing but I think she gave up after being beaten in celebrity race by much unloved Tory MP Matt Hancock (who is now in the jungle on I'm a Celebrity)
I go to Cheltenham Gold Cup day every year and one year she rode in the amateur race that follows the Gold Cup. It's a proper high profile race with very experienced amateur jockeys, some of whom have won big pro races. And she came pretty close to winning - third I think.