No answer (although at this point there probably won't be any answer coming in regardless), sorry.

I'll just provide some thought behind this working, and you can check during your tutorial whether this thinking is completely off.

First of all, I assumed that 600W is total power output rather than useful power output.

This means that on flat ground, 600W is the amount of power required to maintain the KE and do work against friction over some period of time, t1.

Logic is similar for the slope, 600W is the amount of power required to maintain KE, gain GPE and do work against friction over some period of time, t2.

t1 and t2 are almost certainly different. Not sure whether d1 and d2 are different, and this results in 6 unknowns (d1, d2, t1, t2, friction, speed) but only 2 equations.

Now this is almost certainly wrong, so this post is more or less useless in that sense. It's more of "looking forward to an actual solution in a comment" kind of post.