I would suggest working on distance.  As a result, you will also find yourself getting faster.  How far you go depends on your goals as a runner.  If you want to stay fit and run the occasional, recreational 5K or 10K, 15 to 20 miles per week should be enough.  If you want to be competitive or run longer races, you'll probably find 25 or 30 miles per week about the minimum.

One plan is the "One hour runner".  There are several versions but they're all similar.  One is on the 2nd half of this site: http://www.kathrineswitzer.com/fitness_run.html.  You can google "one hour runner" for more.

Another possibility is Bridge to 10K (aka B210K).  You can google this one too.  One source of the plan is here: http://www.sparkpeople.com/myspark/groups_individual.asp?gid=46058.  Some dislike that the plan returns to run/walk "intervals", whereas One Hour Runner is steady running.

Len