Op, the reason you get these responses is that this is done about twice a month.

To answer your question, the apparent paradox is that the way to increase your average speed is to forget about average speed.

To get faster you need to do intervals (which are going to lower your average, but maek you stronger and faster).

With 800 miles base, you're ready to start doing some interval training. Pick up Friel's book, or one of Carmichael's.

Or do a search for interval training.