When it comes to running, especially over long distances, it's universally accepted that hills are the hard part. Running uphill sucks! But for how long? Typically, as long as the hill lasts, and then a short while for your breathing and heart rate to return to their pre-hill levels. When we run uphill, gravity provides less assistance than it does on level ground, or especially downhill, and we have to produce more force with each stride just to keep moving. Add to that the fact the speed is relative to the flat ground distance you have covered, and doesn't account for the added distance created by the incline, and it's easy to see why people rank hills as the worst bit of any run.
I have a suggestion though; we consider downhill sections as the worst. Wait a minute, where gravity assists most with forward travel, force production required is lowest, and heart and breathing rates stay down is where I want to focus? Yes.
Muscles can contract in 3 ways; concentrically - when they exert force as they shorten, isometrically, when no change in length exists, or eccentrically - when the muscle lengthens under tension. One way to think of eccentrics during a run is when the body absorbs the force of landing. This force is largely dictated by your body weight, and much of it can be retained and used to contribute to the next stride through elastic energy known as the stretch-shortening cycle. This action becomes increasingly more demanding when we run downhill. Gradient is critical here - the steeper the decline, the more we have to work to absorb and reuse forces generated on landing. Typically, we see longer ground contact times, and less efficient use of stored energy with each stride. This is somewhat masked by the contribution of gravity pulling us forwards, but the critical feature of eccentric muscle contractions is their ability to induce soreness, especially Delayed-Onset Muscle Soreness, or DOMS.
Have you ever done hill sprints or hill reps? Sure they suck at the time, but because they are predominantly concentric in nature, there's often little soreness the next day. Run a lot of downhills though, and the quadriceps, especially the part just above the knee known as the Vastus Medialis Oblique, or VMO, can be really tender for days. Walking upstairs afterwards is okay, downstairs can be agony!