Hi Leah! The short answer to your question is it didn’t take long, and I did it by doing short 12 minute high intensity body weight exercise routines as well as running.
The longer version is, it didn’t take long, and it actually happened quite by accident. In high school my best time was 8:45. I was young, very active, and of course had barely any body fat. I never ever imagined I would ever run faster than that.
When I started running again a few years ago, I tried an interval technique. I would run at a steady speed for four minutes, sprint for one minute, then run another three minutes, sprint again, and run for one last minute before finishing. I only ran for ten minutes (sometimes longer, but never at that pace for longer than 15 minutes).
I would just always push myself. I would make sure I was sweating like crazy and really barely making it. After ten minutes was over I dripping sweat. It was a hard ten minutes, but it was only ten minutes. (If I try to talk myself into committing to a longer work out- I often won’t. This was a compromise. “It’s only ten minutes.”)
I started doing body weight workouts at home from Zuzka Light via her old Bodyrock.tv website. After just two months of doing 12 minute workouts at home and running occasionally, I came back and tried my old run and much to my surprise I was flying. I didn’t realize how fast I’d gone until I looked down at the timer. By doing the core training my body was just better prepared for the run.
I hope that answered your question (and didn’t bore you to tears)!