In the workout app, What is the difference between an outdoor walk & and an outdoor run
Based on movement + age, height, weight, heart-rate it should be able to compute calories burned. Right?


Not to a great deal of accuracy. A fit person's heart will tend to beat slower than an unfit person's heart even if they're the same age, weight and height.

Knowing your stride length will improve the accuracy. The page mentions calibration. The walking calibration may be kept separate from the running calibration.