Yesterdays run was nearly 8km, 192m altitude, where most of it was on good (very enjoyable cycling where you don't lose fillings or have too keep a eye out for pot holes) forest fire roads, legs hurt. Today's nearly 3km on a flat forest hard packed "smooth" gravel, think park track where you can easy push a pram, with only 19m of altitude, legs hunted more. Same happened a few months ago, when I tried to run around on the streets, since no tails were nearby. Other than boring me to death the road run was really painful for a few days afterwards.
Where previous runs like the nearly 10km, 715m altitude, run on Pyrenees mountainous tracks, or any of the 5km and 250m+ on narly, rocky, where is the track, roots, scree, trails, I think I found the track again, sheep, up and down mountains in and out of deep forest tracks. Legs are a bit tired a wee rest and some water later I'm walking normally.
But the flatter/uniform/smooth the surface it the more painful the legs are. From ankle and about 6" up all around is painful and even after stretches they are hurting for a good day after. And if I forget to move the legs they are rather sore/painful when taking the first steps after a sit down.