Posted: 10/01/2016 at 18:23
I think you should try a running max test.
The only thing I can say is that HR does increase with even the slightest incline, increased heat, humidy, dupoint, fatigue, headwind etc.
I do know from my own experience that HR does force you to run slower than you think -partly because our perception is usually 'off' and we habitually run at a moderate intensity when we think it's easy intensity.
I can understand your pace at 70%, but 80% should be looking and feeling like proper running, albeit slow