I did 21.8K yesterday. My legs were stiff. It felt as if I had 2Kgs on my quads and 1Kg on my feet. But at the same time, It had rained hard the previous night and I had quite some mud sticking on my shoes. Anyways, it wasn't hard cardiowise, just speed-wise. I am just glad I didn't have to run on the tarmac the whole way.
I will see if I can do something about speed this week. I biked this morning and the bike said I had burnt 600 calories.