Not sure what you expected to get with a question like that. Seems like the range of answers was limited, obvious, and things that you already knew. May not be popular to go up against Captain Haddock, but I?ve always found XC skiing to be effective cross training, especially classical style. So, apparently, did Kampenainen, who cross trained on a Nordic Track. I also did some running on snowshoes back when they were those big wooden things. Since I was a track guy and not a marathoner, I had a particular hill I sprinted up over and over. It was a dead end road, real steep, with no traffic. Low impact due to its steepness. Repeat until fully fatigued; then do 3 more.
Others have talked about running on concourses, parking garages, etc. Even if they have a linoleum tile on top, they?re concrete 1/16? down. Doesn?t sound promising for a guy who gets stress fractures.
There?s a story about Dave Hemery, 1968 Olympic champ in the 400 hurdles. He was an Englishman at Boston University. Sometime in the winter of 67-68, Christmas break, I believe, everything in Boston was closed due to a huge snowstorm. He hadn?t heard that his practice with the coach was cancelled, so he made it down to the locker room anyway, expecting to find that the coach wasn?t there. He was. Hemery said something to the effect that they of course wouldn?t be training that day. The coach?s response was to open the door out of the locker room, and point at the track, and say, ?The road to Mexico City is out there.? Hemery did the practice. The rest is history.