If your goal is a Zone 1 workout, but you exceed the upper limit of that intensity, then the stimulus of the workout changes. In general, it’s best to stick to the planned intensities so that you’ll get the desired response from the workout.
With respect to the heart rate data, it’s best to work off of the maximum heart rate that you achieve rather than the average. This is for the same reasons mentioned above. For example, an average heart rate of 150, with peaks and valleys of 170 and 130, will be a very different workout than one with peaks and valleys of 155 and 145.
For this reason, it’s normal to have to change from running to walking if the demands of the terrain increase.
I hope that helps.