I was involved in a cross country race once, that went from Seattle, through the Tir, then onto Phoenix. Each "leg" had stopping points, and whoever came in first got that much time of head start on the next leg. Each team had "bodyguards" (which the players served as - one was actually a racer) to handle protection and to make things go well. Of course, if a team was caught tampering with the other's cars, team, or driver, they were disqualified. Caught being the key word!
It was really fun, it probably took like 3 sessions to do the thing (maybe 4, don't remember), but it was fantastic. The team member that served as the racer had surgery to make her look like a racer we were attempting to protect - who was getting threats to not race. We had to protect her, our member diguised as her, and be timely on all aspects as well....which was really difficult, because the team wasn't in a race car. So during the race, while we were in our trucks, suv's, etc, real damage could be done.
My suggestion for a third leg of the race is the desert. Not only do they have the wear and tear/length of the race, but they have hazards of weather too. Check out Hazard Pay for info on deserts. Another idea that just came to me was to have a scavenger hunt type of race; kind of like the reality show "the amazing race."