The Samba Bus challenge takes a very fine balance of agression and patience when driving. It's very tempting to try and jump ahead several cars at the turns because the AI drivers slow down to a crawl, but then, if you hit the wrong pylon, hit a bus (or a bus hits you) you get DQ'ed.
Because it does seem like it takes forever, you actually have a decent amount of time to get to the front. I think it's important to be more agressive in the beginning in terms of passing other drivers, then start dialing it down a bit on the 2nd lap.