Run from the W or St Regis along Chang'An Road to Tiananmen Square, run through/past the square and past zhongnanhai (the leadership compound) head north after zhongnanhai running next to the big red walls until they end then turn back east and run along the road behind the forbidden city until you're past the forbidden city, then turn south until you hit Chang'An Road again and follow it east back to the W or St R. The whole route will have wide sidewalks and lots of sightseeing. Should be between 12-15k.