This along with the other 2 achievements for watching upto 44 hours and 44 minutes only count time spent watching and not paused in video or in the main menu.
Unfortunately you cannot speed this up by fast forwarding as the app only counts actual time watching! Of course 4oD (Now All4) is also one of those apps that just loves its ads and they do not seem to count in your watch time either.
Sorry to be the bearer of bad news there but I hope by writing this I answer the questions you have that I too had regarding what counts but you can snap the app and do something else instead which is exactly what I suggest doing, find a long video and snap it you can even watch the same episode over and over.