Extra food- depends on how much food you take for each day. I have my food pared down so that I end just about every trip with no food- and often no food the last day. If you often have some food left over, then perhaps you already have your "emergency" food with your current food plan. I take 1.25 pounds, 2200-2400 calories per day. If you are taking close to 2.0 pounds per day you probably already have enough for a day's delay. In dire need,
you can sit on Muir Pass and beg for food off all those PCT-JMT hikers who have taken too much!
You also have to consider your own body's reaction to low food. If you tend to really loose energy when low on food, be careful. If on the other hand, you have a bit of extra baggage around your waist when you start, you may be OK. Also, most people tend to have a reduced appatite the first few days. By about Day 10, you could be really needing lots of calories. I have been on several 4-day survival treks (45 miles) so am well aware of how my body reacts to starvation. Have you had similar experiences? The thing is, if you are behind schedule, you will need to make lots of miles in a day, the very situation that requires more food.
You can also pack more calories per pound in your food with careful selection of "calorie dense" food. Remember that it is not the pounds of food carried, but number of calories carried.