Record steps or amount of exercise

I am not sure which way is better. But MFP is designed to use your fitness tracker to gauge your activity for the entire day and determine the calories you burn. When you put in exercise, it calculates the calories based on the type of exercise and duration. To avoid double counting calories, it ignores the fitness tracker data during the duration of the exercise. I would suggest that if your only exercise is walking, then you can just not track your exercise at all and let your fitness tracker and step count load automatically. But I will also encourage exercise beyond simply walking - it's a great beginner exercise for sedentary people, but higher intensity and more varied exercise will be more beneficial in the long term.

Assuming you have a fitbit, there is a fitbit users group that can give you more detailed information on how MFP, Fitbit, and your exercise all work together.