I am -very much- not an expert but I suspect the difficulty profile is more like a hump than an upward slope - going downhill works different muscles than uphill and frequent* switching between the two seems like it would put more stress on you than uphill[hours]+downhill[hours] due to having two sets of muscles in use, less time for recovery, less time for warming up, etc.

(This is where we need a real physio / doctor / anatomist!)