RJC - I was going to recommend the Jack Daniels book above. With the paucity of running training you have done, to run those HM and full mara times is really impressive. I'm probably twice as old as you are and can run faster, but I need to put in a hell of a lot of training to do that. So I'm quietly envious as I pursued so many sports when I was youger, that I never excelled at any one.
In a decent running club with some good coaching and a lot of determination on your part, you could become seriously good.
Read the JD book and get to understand the physiology of running training. It will make you far more effective at improving in the future and avoiding injury. Then you'll be able to understand what this means. Your VDOT value is 49.
Good luck.
And I'll offer one other bit of key advice for starters. Get a gait analysis done at a specialist running store that has an outstanding reputation in your area. Then you can be sure you have running shoes which are suited to your body and running style, which will let you train harder, run faster and avoid injuries.