Totally agree with this, I started at 34 after being a 20 a day smoker, trained for a few months and dragged myself round a 10K in 55 mins, didnt think I could get much faster and mixed it up with cycling and as I got fitter from both, times started to come down. Year on year consistent training has done it, sadly I get injured a lot but can usually drop in a couple of good performances a year, managed a 1:15:30 half last year, ran 2:57 this year despite jogging (and stopping for over 5 mins combined) the last 10 miles. Have a PB of 2:45 but think if I can finally turn up to one of these races fit I might break 2:40. At the height of marathon training Im doing 10 hours a week, 3 in one run and maybe 1.5-2 in another and the rest made up of mileage here and there and a lot of it at easy pace with a couple of hard sessions a week (or races, not both).
I've just been out on a 13 miler to test my injury, seems okay actually but man have I lost fitness in the last month or so, London at the weekend is gonna be tough even if I shut it down as I just dont have the conditioning. Also starting in a pen (I'm on the front of the blue wave so actually first time I've run out that way, I'm usually on red which has a different route for first 5k) where everyone is planning to run an hour faster than you is kinda embarrassing, I reckon Im gonna have a couple of thousand people pass me.