The brilliant thing about running is that it's a very honest sport. If you keep going you will get faster. It's as simple as that. You need to run 3 times a week to get better. And once you've got a firm base then you can up your distances, and push your speed if you want to and this will help you improve faster. You don't have to run every run fast to improve though- I think a fair few people who start running make this mistake- running slow and long helps fitness too and then you can run 3 or 4 miles at a faster pace to help your speed. Main thing is to do it though and enjoy!
Well, if you can run 5 miles at any pace you are not unfit as most people would term it. How fit you are is very relative though. You're fitter than a lot of people your age, but does that make you fit? If you look at the stats for runners your age re age grading you'd be quite low in those, but that's because a lot of them are folk who've run for years and are very fit.
It's fab to be running and to be able to do what you can do. Get your times up to 10 minute miles and you'll be fitter. Stop running and you'll be less fit, quickly. Keep going is the main thing!