In high school, I would run 15-minute miles in gym glass because they wouldn't let us take a shower afterward, and I didn't want to be sweaty for my next class. (For context to those outside the US, schools measure students' athleticism by how quickly they can run a mile, or ~1.6km. 10 mins or less is considered good.)
For years after high school, I didn't run, not even on a treadmill (but did weightlifting to exercise). Then sometime in the last couple years, I started running the gym track after weightlifting sessions. At first, I could only run around the track 3/4 of the way, then walk the last 1/4, and repeat two more times for three total laps. I'd feel pretty tired afterward.
Then I could do this for six laps, where I would walk one full lap on numbers 3 and 6.
Then I increased this to 12 total laps, where numbers 3, 6, 8, and 12 were walking laps. This is equal to a full mile on my gym's track. Shortly after this, I participated in a 5km race and completed it after something like 45 minutes, so about 15 minutes per mile (this was with actual effort, unlike high school). That was last June.
I signed up for the same 5km race this year, and for the past couple months have been training for it two to three times per week, following the same 12-lap cycle described above. Last month, I was finally able to cut out one full walking lap. So it followed:
(3/4 run, 1/4 walk) x 3
Walk
And repeat two more times for 12 total laps. Then today, with much pain, I was able to run three full laps before having to walk. So:
Run x 3
Walk
(3/4 run, 1/4 walk) x 3
Walk
(3/4 run, 1/4 walk) x 3
Walk
This brought my mile time down to ~12:30, which is still not great, but should cut about 8 minutes off of my total race time. It doesn't seem like much, but after two months of training and two more years working up to that, I'm proud of the progress I've made so far. My next goal will be to run three full laps, walk one, and repeat to make 12. By the time of next year's race, I hope to be able to run 5km in under 30 mins!
Now if I can master the art of music hacking, I'd be even more proud. 
