So true..
This weekend I ran an 8 miler on gradual but consistent and challenging hill trails. SO hard but oh so good.
Years ago at a rough time in life, I began tackling a road - Sulfur Mountain Road - that was all switchbacks, up up up for six miles.
The first day I could only run about 100meters at a time. Gradually over that summer I was able to run it, nonstop to the top, at 9 min miles with a 12 lb pack.
Sure, it's nice to cruise the lazy flats, and there's joyous thrill of racing the downslopes. But hills, that's where the real growth happens.