Trust me, the last thing you want to do is waste your time inputting the time you were sitting, standing, walking, watching tv, etc. In my experience what the watch detects is pretty accurate. As indicated above sometimes it thinks your sitting if you haven't moved around too much and sometimes it thinks your standing if you've been doing a lot of gestures with your arms. But it's already doing as best it can based on how you are moving your arm.
The red "move bar" graph in the activity app gives a pretty good approximation of how active you've been. If you want even more information (or information visually displayed differently) there are dozens of great apps which track that data in different ways.
