Heart rate training, a new way to work on speed?

As I have recently gotten a watch that tracks heart rate, I was interested in finding out what heart rate training could do for me.  I consulted a few friends who had done heart rate training before and was sent a few articles to read.

http://www.runandbecome.com/running-training-advice/the-secrets-of-low-heart-rate-training

http://home.trainingpeaks.com/blog/article/joe-friel-s-quick-guide-to-setting-zones

Both pretty good reads... so basically my goal is to run 1 or 2 runs a week with heart rate zone 2.  Usually this will be my long run unless I have a speed training goal set for my long run.  My first long run that I've done so far I kept hitting zone 3 a lot.  It was really hard to pull the pace slower in order to stay in zone 2.  For me that is about 143 bpm or maybe as low as 133... a small range to try to stay in for many miles.  The other thing I noticed was at the end of the long run, I am more fatigued and keeping in zone 2 becomes harder and harder.

I did notice something that improved though.  My faster runs I seem to be improving cadence and getting a lower average heart rate.  I've been trying this for about 3 weeks now.

So this is a work in progress and I'm hoping to see if there is any real impact over time

As far as training, I do have a half marathon coming up in April and a few smaller races in between then.  I'm taking it as it goes and trying to be reasonable about my expectations.