Mine would be to trek atop the whole stretch of the Great Wall of China. From the China Sea to the Gobi desert. Maybe when China gradually opens up they will allow "thru-hikes" but for now we'll have to be contented with 5 km and 10 km "marathons".

http://www.great-wall-marathon.com/

Any other daydreamers out there?

You might check out the book "Alone on the Great Wall" by William Lindesay. He pretty much ran/walked the length of the wall (while trying to avoid Chinese officials).