a constant 8% slope will add about 3% total to your run and 8% is pretty steep, so on normal hills--say 3%-- you will barely add any.
If I show you mine, will you show me yours? I'd be really interested to see the math that takes an 8% slope and returns a 3% difference in distance.
An 8% slope would be 8 (feet/meters/yard/cubits...) of rise for every 100 (feet/meters/yards/cubits...) of run.
Doing simple Pythagorean solution of a^2 + b^2 = c^2 where a=100, b=8, gives us:
10,000 + 64 = c^2 (where c is your distance actually run up the slope).
Solving for c gives us 100.3, or a difference of 0.3%, not 3%.
This means for every mile (5280 feet) measured on the flat when actually running up a steep (8%) slope, you actually ran 5297 feet, or a stunning 17 feet longer. 10 mile run? That's 170 feet difference. You sure you can map anything on mapmyrun to that level of accuracy?