How many miles per week should I run to get faster?

I'm a very casual runner. I run to keep my metabolism going. The last few times I went running I decided to time my mile on the gym's indoor track, six and a half laps equals one mile. My mile times are under six minutes. I'm probably putting in 4-8 miles every week of total running. I don't do anything else, exercise wise anyway, except a little walking. Time trial 1, last week, 5:57, time trial 2, this week 5:54. Time trial 3. yesterday 5:58. After running each one I was pretty tired and out of breath but it felt good.

I want to know if theres any way I could get down to sub 5:00 or even just 5:30. Some people have told me I need to work up to 30 miles a week while others have told me to just work on my sprinting and raw speed. Side note: I smoke, half a pack a day roughly of cigarettes. Obviously I might want to quit that if I want to get faster, or at least cut back. Been smoking for the better part of four years. Kind of looking for something that could take the place of that habit.