This is a great idea! I had a similar one that I was going to post about. The idea is along the same lines except with checkpoints instead of a circuit. At every checkpoint you reach you get the grid ref to the next checkpoint. It would be up to you to plan the quickest way. Maybe even have a co-drive to help you!
Can i suggest the idea of a Le Mans style start? Where the vehicles are lined up and everybody has to run to them once the race starts?