Coyote Montane posted a summary of his decade showing how NOT to decline through your 50s. Here’s the corresponding summary of my best race times each year over the past decade, showing a lot of decline even though I have kept trying.
2008: 5K 20:58, 10K 41:42, HM 1:37:27
2009: 10K 44:56, HM 1:37:40
2010: 5K 21:08, 10K 42:46, HM 1:37:14
2011: 5K 21:42, 10K 44:00, HM 1:35:42
2012: 5K 22:31, 10K 47:32, HM 1:42:15
2013: 5K 22:19, 10K 47:07, HM 1:42:15
2014: 5K 22:53, 10K 47:57, HM 1:42:28
2015: 5K 22:41, HM 1:40:17
2016: 5K 22:59, 8K 36:57, HM 1:40:56
2017: 5K 23:18, 8K 37:42, HM 1:43:07
These were my best races of each year. I ran between 3 and 11 races each of those years and some of my times were much slower, but this is a reasonable summary of the best I was capable of running during each of those years. Over this time my weight was reasonably stable (114-118 at a narrow-framed 5’5”) and no big changes in my diet. Lots of injuries, but all except the most recent (Achilles tendon on July 5, 2015) have been recoverable.
I live in an area where there are quite a few races within about a one-hour drive, but the level of competition is generally low. Local races are geared toward participation, not competition. As a result, even when I run rather poorly I win my age group most of the time, and if I don’t win I’m usually second. For example, my most recent two races were both slow, a 1:47:57 HM and a 24:25 5K. I won the F55-59 age group in both races, with 23 competing in the HM and 72 in the 5K. If I’m willing to drive more like 2 hours to the Sacramento area or the east bay I can find many more races with a generally higher caliber of competition. I do that once in a while to remind myself that there really are women my age who are much faster.
On to age 60 next May!