I just looked, and the running time is about 301 minutes, including both sets of opening and closing credits. When it was televised in 6 episodes, there would have been extra time for the opening & closing credits shown for each episode.

When we speak of a 6-hour production, we mean a 6 episode mini series with each episode nearly one hour in length. In the U.S. P&P2 was shown on A&E, and commercials took up the additional time within the hour (and a little more, as BarbaraB & DeborahAnn have pointed out, resulting in some additional cuts from the program). But the dvd version (and the purchased VHS version as well) have the full production.