You really got to make sure your planning your route as you go, i.e. look for the route which has the most cars along it and always leave the multiplier to tick down some before you go for the next car (see diagram below). This gives it more time for cars to respawn down the streets and around corners. I have done it twice just running round one block with fresh vehicles spawning every time.
Someone asked for a diagram as they didn't fully understand my guide so I am posting it here as well as it may help some others.