There are some stretches where you have to carry food for a few days just to be safe. The most desolate stretch is the Casiar Highway in Northern British Columbia, at one point you'll need to carry enough food for four/five days. I always carried roughly three days of food to stay on the safe side, but usually it wasn't required. Get a good map of those areas and see what towns are along the way. For the most part all towns will have at least some kind of small shop where you can buy stuff.