Hi.
I am a slooow runner also - after 5 years of running pretty faithfully, I run a 10:15 race pace (10 miler). I started at 12:00, so I have basically only improved 15 seconds a mile a year!
That being said - the only thing that has really improved my time, I think, is speedwork. I hate it. I really do. But pushing myself in terms of speed is the only way I get better.
As I was whining about this one day to my husband, he said: "well, duh! Of course you have to push yourself hard to get better. Running easy all the time is like lifting 2 pound weights and expecting to get stronger."