I found the inclusion of tempo and speedwork session into my training increased my pace massively.
I have a 405, and seeing my pace and related HR over time was a great motivational tool, although without knowing your true max HR (or at least roughly what it is) then HR is a useless reading.
Drop at least one of your runs in favour of a tempo or speed session, be prepared for it to be tough, and hurt, because if you have been used to "cruising" then your body ain't gonna like running in the red, I can remember so speed sessions where I have been close to weeping on the final rep!!!
2 miles at 9-10min mile pace is a good start, speed session are generally (for me) reps of 400 or 800 metres, with a 250metre slow intervals and 1 mile warm/cool (totalling about 5miles), the 400/800 metres is done at about 95-97% effort.
A general tempo session would be 5 miles, with 1mile warm / cool, and 3 miles at a fast pace, these sessions took my 5k PB from 34mins to 27mins in about 8 months!
You've got to remember, that by doing long slow runs all the time, you'll become a long slow runner, you need to push it to get faster.
Edited: 31/03/2009 at 13:39