1. You need a word of warning abou this achievement. Game counts all race attempts as races driven, if you for example play on higher diff setting, restarted a race like 10 - 20 times after crashes etc. you will end up with much more races to drive to get 50% win ratio...
Because of this I ended up with over 500 races driven in gokarts and have to win hundreads of them to get progress bard to >50% wins ratio.
2. If grinding this in quick race weekend backing out to main menu after each race is not necessary, you can just restart race after you finish it.