I think I can relate! I'm also a naturally skinny woman who started working on building up my fitness in my early 20s - first with running and now more with lifting. I have managed to stay relatively active throughout the ten years since, but sometimes do have lower activity periods when other things get in the way. After such a hiatus I have noticed that when I do ramp back up with the exercise I am able to get back up to my "fittest" levels and rebuild muscle much faster than the original effort. Now I am putting an effort into doing more running along with lifting again for the first time in years and even though I'm still not as fast as I used to be, I'm surprised at just how quickly the performance is improving, and how quickly my running form falls back into place.