I am 51, had a PR of 32:12 at age 20. I took 10 years off prior to starting running again in 2006. I started at zero and now am hovering at 18:47 for a 5K. However, I've taken it slow and have gotten progressively faster over the last 5 years.
Here is my personal list of impediments to running much faster:
1) Mass. It is very difficult to stay lean. I am 150 now and raced around 142 in College. So, I'm close, but not quite there.
2) Injury. By trial and error I’ve found I CAN stay healthy (No injuries now for a year) but I have to be smarter at avoiding injury. Like adding superfeet inserts, or altering my training to allow more rest. Being more aggressive and proactive in treating injury is critical. I was very poor in treating my own injuries in my 20’s. The answers and solutions are out there if you go get them.
3) Reduced metabolic efficiency: read; some minor restriction to blood flow throughout the body occurs with age. My VO2Max is not high as I cannot deliver O2 as efficiently as before. To simplify: we all clog up a little with age, deal with it.
4) Motivation. This is the biggest. I have a very active life and running is just a small part of it. It’s not as important to me as it once was.
5) Fear. I remember in my late 20’s I’d let myself get way out of shape. I would just bust 10 a day and I got it all back in a month. I can’t do that now, the risks of injury are higher so I scale back my training. I don’t want THIS injury to be the last one…EVER.
6) Social expectation. Our society does not expect or think we can run as fast as we already are! This is a powerful force. It’s hard to believe in yourself when your peers are; dead, obese, nay-sayers and critics.