Building your endurance base with lots of slower aerobic runs is the best way to get faster as a runner. After lots of trial and error, I learned this secret of the more advanced runners. My 5k times had plateaued for months. So I got a heart rate monitor, determined my maximum aerobic heart rate, and ran religiously at that slower aerobic pace for 3 1/2months; no tempo runs or speed work of any kind. Then I smashed my personal best 5k by over 40 seconds in the next race I ran.
See http://philmaffetone.com/180formula.cfm for his aerobic training hear rate formula.
