I have sort of walked on the sumo ring in the Tokyo stadium -- it was covered with a strip of red carpet at the time -- when I was one of the many folks who got to clip Musashimaru's hair at his retirement ceremony. It didn't seem any harder than any other floor.

The sumo training sessions seem really, really tough to me as well, way worse than the matches. For example, in the corner of the practice room there wasa round pole about the same diameter as a telephone pole placed solidly upright in the ground. The exercize was to run at full speed and smash against it with your bare shoulder. Amazing!