Great thread...as the word falling off a cliff seems about right.
I will be 49 in a couple of months, and was running really well until about 46-47. It was weird. I PB-ed in the marathon at 37 (2:35 at NYC), ran 1:11-flat half at 38, and even ran 31:40 in the 10k at 39. The next few years I declined, but still could pop sub 16 for 5k at age 43, and 1:14 for the half at 44, and a 30k in 1:49 at age 46...then at age 47 after a slight Achilles problem. Boom...it was like my old self was gone. My tempos at age 45 were comfortably 17 flat for 5k, became rigorous at 18 mins (seriously killing me.)
And I feel the same! My mileage is about 40 miles a week....It was so bad, that I needed to shift things up. So, I started hitting the weights (3x a week for 45 mins), and I changed my diet about 6 months ago. I have really increased my fat intake (almost a Keto diet- 60% fats, 25% protein and 15% carbs--carbs only after dinner) to jack up my cholesterol (the precursor to testosterone). And it seems to be working. My bench press is back over 200lbs, and my calf raises are back to 350lbs (like in my 20s). My running is still slow, but I managed a 17:30 5k tempo last week and it didn't kill me.
Not saying this is for everybody....but diet and an extra day of rest makes one feel a lot better (for sure drop to 2 days a week of hard running)
ps///I also noticed the booze thing too. I have dropped the ability to throw 'em back (but I still like one or two).