I used to run for fun. Always, for tirelessly very long lengths of time. Up and down hills and on all types of terrain. In rain, wind, sun, heat, snow, it didn’t matter. Now, I’m particular about the terrain, temperature, and distance. Why? Because my knees ache from the surface or the climate or the distance. Again, because I’m older and I’ve been running for over 30 years. My knees don’t like running, but my head and heart love it.