Total distance covered is the major factor in how many calories you burn.
There is no information in your post about speed (or by implication, distance covered). As a slow running speed, 300ish calories could be feasible. At a faster running speed (and thus more distance covered) 400 calories could also be feasible.
Walking and running are pretty well studied activities, and online calculators tend to be fairly reliable for these. HRM's come into their own for activities where it is hard to objectively judge the intensity. This doesn't apply to walking or running.
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