M65 here. There is a reason the 50+ thread has been in existence for 13 years. And it even existed several years before it was saved from disappearing each week. The vast majority of runners turning 50 notice something huge happens to their body around this time.
Here are three things I’ve noticed happening as you near 50:
1) Your actual muscle fibers become weaker. Is it hormones, cellular metabolism? I’m actually not certain. I keep detailed records. I had certain workouts that were predictive of performance in races. First I noticed I COULD still hit the workout but it no longer predicted the race time. Before 50 I could always over-perform in races better than most workouts predicted. Soon though, I could neither hit the workout nor perform in races. This hit me at about 51.
2) It took longer to recover from workouts. I found myself repeating “If I could just freakin train I’d get faster”. But that never happened. I could no longer hit those hard workouts twice or even three times a week.
3) My injury rate skyrocketed around 49. I seemed to stay injured all the time. I had a devastating heel spur at age 53 that took over 2 years to heal. In truth, my racing body NEVER got over that.
Here is the most disturbing part of 50+ running. A very large number of people simply disappear from the running scene.
If you look at websites devoted to masters running you will see folks who are performing at very high levels. This is a huge statistical anomaly where only the extreme outliers are represented. For every 90+ age-graded masters you read about there are thousands and thousands who dropped out of running. So you only see the extreme end of the spectrum and it is grossly not representative of the reality of running after 50.
A caveat: you may see in some popular running magazines statistics showing more older folks running. The majority of these folks are not competing. They are participating. I mean, that’s actually great, BUT, it does not give us any data on actual athletic performance of older folks.