Follow the protocol the Ortho should have given you. Don't slack. Do the CPM if they gave you one. 6 hours (or more) a day sounds like a lot, but if you break it up into 2hr chunks and read/watch TV/troll internet forums, it goes by quickly.
Flexion is important, but for the first couple of weeks, extension is even more so. Make sure you work on getting that leg straightened out as much as possible. Put your heel on a pillow when you are sitting or laying down so that your leg is extended as far as possible. It is much easier to work on flexion later on than it is to work on extension.
Best of luck with the recovery.