I know I'm replying very, very late to this but if you're not horribly averse to using Cheat Engine there is a table out there that has an option to modify and/or freeze the time you have left in any of the 'countdown timer' missions. Do a Google search for "witcher 3 cheat engine time remained" and you'll find it. Can't verify if it works for every version but I just used it for GOG GOTY 1.31 to freeze the timer for a certain other "timed" quest (you know the one, where a bit more's at stake than a racing trophy) just so that I could check out all the possible interesting sights in said quest without worrying about the timer running out. Use this and you should be able to leisurely trot your way through the entire race without bothering with a single dummy or target.
