Apart from gathering mushrooms, there isn't another food source until you get the next level of farming technology. If your population is higher than about 15, you will have great difficulty feeding your tribe with only the food sources you have now. The crops are on a fixed schedule, and won't grow faster now matter how many farmers you assign.
Running the game at 1/2 speed will help conserve food. If it gets really bad, you can pause the game and mushrooms will still appear allowing you to build the food supply.
