I see from the map attached that the grade of this trail becomes less near the top. That allowed your pace to increase and give the negative Pa:Hr. So this Pa:Hr ratio is no longer going to be useful for the AeT determination. This test must be done on a grade that does not change. So either a flat course out doors or what we suggest for most folks, a treadmill with constant grade and speed. Most trails will have too much variation in grade to be usable for this.