I was a fairly decent U20 middle-distance runner with bests of 1:56/3:59/4:18 (800/1500/Mile) in 2000 at age 18.
I then gave it all up during a gap year before University, getting into drink, partying and even smoking. Despite occasional shortlived attempts to get back into running, I basically didn't run seriously again until late 2008. I quit smoking for good, joined a local club and decided to see what I could still do.
I gradually built up to around 50mpw with two sessions and a long run and went on to run 1:58/4:03/8:53 in my first track season since 2000. After another winter, despite a 6 week break for a hamstring injury, I managed to drop my 1500 to 3:57, breaking my 10-year-old PB. I also added a 1:57 800 and late-season 8:49 3k. Since then, I have struggled with various injuries. Nevertheless, my experience taught me that you can always surprise yourself if you get back in to dedicated training. It is easy to think you will never regain your previous shape when unfit, but with a few months of training the body begins to feel good again.