Dont worry about it like that. You dont have to walk or run after you eat a meal. Your body has something called metabolism that does the work for you. Just stick to healthy eating, exercising, and you'll be fine. Slowly but surely.
walking after a heavy meal is good way to stimulate digestion
other that you can also sit in a posture called 'vajrasana'(sorry dont hv the english word for it make a google img search, you should find it)
the yogic posture stimulates blood flow to the stomach thereby improving digestion.
but relax nothing just converts to fat that way
as mentioned by 'Tonymcclellan' stick to a healthy diet and exercise
It's not turning in to fat. In fact, you're burning extra calories as your body digests your food. Eat good healthy foods and focus on the good they are doing for your body.