The HRM is the best advice, as you have to do the least work.
If you know your average walking speed, and average running speed, you can make two entries. Eg if you walk 2 mins and run 3 mins, then work out your entire workout. If it was 30 mins then it was 12 mins walking and 18 mins running. You can enter those times at your average speeds for two different calorie burn statements to cover the entire workout.
Deb, in New Zealand