Beanie, have you measured your max HR or is the 155 figure from an article?
According to calculations my max HR should be 175 or there abouts, measuring during exercise I've recorded 193, a fair difference. As HR zones are calculated as a percentage of either working or max HR you might not be running at the best HR figure for yourself.
How are you measuring your heart rate when you run? Are you taking a spot reading or an averaged reading over an interval (either mile or time)?
LSRs are meant to be slow, the body carries out its adaptions better at a slower speed than if you ran the long runs at max pace. It might take between three and six weeks to see improvements so keep with it.
I did a marathon earlier this year and am now following a similiar strategy for another attempt in September. I've ditched a lot of my faster running and replaced it with slower running to heart rate and have been able to increase mileage done in each week as not having to recover from so many hard sessions. Leg muscles are developing nicely and I generally run one temp run each week the speed of which is improving.
Results will come if you stick with it.
Cross post with others.
Edited: 18/06/2009 at 16:43