Here is how I usually do it.
I try and grab most of the food early on, especially the hotdogs and burgers. This will make Mari faster and larger, making it easier to get more food down the line.
I try and use rocks to get the out of reach food whenever I can, though it will take a little practice to hit the airborne food consistently.
In terms of points, Size > time. So it's fine to spend a little time back tracking for leftovers.
Finishing fast only gives you 10 points a second, but the more score you finish with, the farther the truck will be launched. Which will almost double your food score.
Your speed when hitting the truck also plays a part in how far it is launched, so just floor it when you get to the final straight!