Welcome to the forum JohnP!
AFAIK, you dont' really constrain a path to a surface. A path is a spline and you would have to manually shape that spline to follow your ground surface. (There is a tool coming in v17 that will conform a spline to another mesh's shape.)
However, the path constraint itself is a fairly basic thing, it moves an object through space along a path, it's not aware of character animation issues like re-posing feet or a character's posture to accommodate walking up or down an incline.
I remember sitting through a Softimage demonstration several years ago where they showed a path constraint that was a bit more aware of this than A:M's. But it still didn't look right. It would not have been acceptable for anything but the most undemanding pre-vis work.
In finer animation most walks, inclined or otherwise, will be posed out step-by-step.