Actually they tended to eat very well.
Even the commoners/peasants ate pretty well, with a wide range of fruits, vegetables, fish, cheese, dairy. Meat (pork, beef, chicken, etc) would be less frequent for the lower classes, since those animals were worth a lot more alive, and tended to be eaten when they were older or injured.
Knights were much wealthier. Depending on the century, they were either below nobility, or were in the nobility. They would have access to meats more readily, and often hunted for sport.
Knights also tended to be in great physical condition, unless they were older/retired. Those who were actively functioning as knights, would practice daily. There's one written account that suggests running a few miles in your armor every day to help stay in shape.