Retraining

I have been running off and on for about 3 years. I really enjoy it and attribute my stellar cardiovascular abilities and deep inner sense of calm to it (HA!) That's actually not true, but it's pretty good stress relief. Since I've been running, however, improvement seems to elude me. I feel like I'm able to get to this point and that's pretty much it.
For my birthday, my husband got me a heart rate monitor, per my request. I recently decided to use this to train more effectively so I found a training plan from a respectable site that has target heart rates for each run. I've been following it for about 2 weeks now and I have been amazed. I have to run really slowly. In fact there are times that I need to walk in order to keep my heart rate within a reasonable range. I have an 8 mile run this Saturday and as it looks right now it may be an 8 mile semi-brisk walk with water breaks.
I'm wondering if this has been my problem with not improving. I've constantly been over training and never working in the range that was actually beneficial. So, we will see. This plan has me working up to 10 miles so I can run the Steamboat Classic in June. I will be interested to see if I improve or if I end up walking the 15K. I know you're on the edge of your seats, so I'll keep you posted.