Criteria for "Exercise" and disability

I have an disability which prevents me from doing most high level exercises (an artificial leg and my 75 years). However, in an attempt to keep fit I am trying to walk at least 1 km each day without stopping. I use the Outdoor Walk module of the Workout app to monitor this activity. However, only a portion of my walking time seems to be recorded as exercise. I assume this is because my walking pace is too slow to always count. I am walking as fast as I can safely manage. And I believe i use as much energy in one my slow walks as a fit individual would use at twice the pace!

Interestingly. the proportion of my walk that counts as exercise seems to vary from about 25% to 80%. I think what is happening is that it only counts after my pulse rate reaches 100 bps. On some days it stays just below 100 for most of the walk - and in these cases I get a lowish "Exercise" ring!

Is there some way to adjust the criteria that determines what proportion of my walks are recognised as actual exercise? Like lowering the pace level?