To a certain point it's get better. For me at the ripe old age of 59, things aren't so fine in terms of running. After 50, things start falling apart - and rather rapidly for some people. My first major injury sustained (Achilles tendon) was during a track mile at an all-comer's meet shortly after 50th B-day. Over the next 5 yrs, I sustained a completely ruptured PTT as well OA developing in three joints. ?Now at 59, I'm hardly able to run anymore. I've known many other runners that I competed against and trained with back in my 40s who facing a similar fate in their 50s. Go check out the Grandmasters thread here and see for yourself some of the debilitating injuries aging runners are suffering.
Too many negative things happen to the body in your 50s. Hormone levels, especially the magic male hormone testosterone, starts dropping precipitously every year. Your cardiovascular system starts declining every year, your digestive system doesn't function as well either. Wrinkles, bags under the eyes, bad skin, etc. all start to develop after 50. You look at yourself in the mirror and say; "WTF...what's going on here!"
Many of us in our middle-age may have our sh*t together - finances are in order, kids were raised properly and are off to college, marraige went well, retirement is now in sight, etc. - but the thing that we were so passionate about is slowly but surely discenergrating right before our eyes.
Young men enjoy your youth in your 30s & 40s because things really go south in your 50s! ?