Originally Posted by Darren
ToH to Tiananmen, it's about 15 minutes (more with traffic).
I would actually walk that (and keep the entire day, with the exception of a taxi to ToH and a subway back from Gulou, on foot). There is usually a lot of activity along Qianmen Dajie and only about 1km separates the north end of Temple of Heaven from the south end of Tiananmen Square. Crossing the street between Qianmen and Tiananmen Square can be a bit tricky; you need to use the east subway entrance and cross under. (To do the same in the west entrance, you'd need to buy a subway ticket).
Just to provide a frame of reference, I recently walked from Temple of Heaven to Houhai (via Gugong and Beihai) in just under 2 hours. I estimate that I walk 2-3 times faster than the typical tourist because I maintain a decent pace and don't stop to look around all that much.
It gets dark around 7 here. As long as you make it into the Forbidden city before they stop selling tickets (around 3p), you should be fine. Jingshan, Beihai, and Houhai all stay open late and are beautiful at dusk.