Going uphill will strain your hamstrings and Achilles tendons with every step. Your pulse and respiration will rise. But as long as you have the breath to, you can keep going at a steady clip.
Going downhill, however, will do a number on your quads—not just with each step, but constantly because even on a smooth slope they will be in continuous braking mode so that your speed does not get ahead of your feet. Where nature has made steps in the form of boulders or tree roots, they never match your foot or leg length, and are as steep as the rungs of a ladder. You wind up grabbing a tree to go one step at a time, sometimes backwards like a real ladder, or sitting on the rocks sliding down one at a time—unless you are as young and agile as a mountain goat. Even then, one slip in a downhill run could see you topple head over heels for twenty or thirty feet which, by the way, would be the only way to make any real time going downhill. If you slow to two miles per hour going uphill, you will be lucky to make one going down.
Satan will always get you when you least expect it. When life is good, when trials are over—for today at least—and we let our guards down, we will get to going too fast, speaking faster than we can think, reacting faster than self-control can kick in. And there we go, tumbling down the hill like Jill tumbling after Jack, who broke his “crown,” by the way. And what will happen to ours?
So when life is easy, when suddenly the ascent levels out or even begins a downward slope, be careful. You can still take a nasty fall that lasts longer than you would have ever imagined.
Now these things happened unto them by way of example, and they were written for our admonition, upon whom the ends of the ages are come. Wherefore let him who thinks he stands take heed lest he fall, 1 Cor 10:11, 12.