[Kaj]

Increase your endurance. From 3-4mi 2x/wk, you want to be doing 4-5 runs per week. 3-4mi on 3 of them at your normal base pace, try to finish each with a good 2-3 min faster pace, do one as a long, slower run, extending the time/dist (say ~6mi), and your final run should be ~3mi at an up tempo pace (anything really would work given your level now -- fartlek, 1 mi ez w/ 1mi fast then 1mi ez, sustained up tempo, whatever, just make it faster than usual). Do this 4 weeks and you'll run 6:45 no problem. Interval speedwork would be a waste of time.