1. Race times. You keep training hard but keep getting slower. I can no longer hold my old marathon race pace for a 5K.
2. Always hurting. I miss the days of being able to bang out a morning 10 miler, increasing the pace as I went, feeling incredible. Today, that same 10 miler is a painful crawl in which the only part I look forward to is finishing so the torchure can end. I would trade every PR I ever achieved "back in the day" just to be able to feel like a runner again. Instead, I feel like an old broken down piece of garbage.
3. Fighting weight gain. In the younger days, diet meant nothing - I could (and often did) eat anything without consequence. Today, if I even contemplate one extra serving, BAM - 5 extra pounds pile on. If I let my diet slide for a full day or two, I'm a complete whale. Maintaining weight is a constant, never ending, and exhausting struggle.
4. Injuries. I can't seem to run for more than 2-3 months without hurting something or another.
5. Knowing that, no matter what I try or how hard I train, I have no hope of ever getting faster. The best I can hope for is to slow down the rate of decline.
The bottom line: young guys, enjoy running now, because it flat out sucks when you get older.