Is there anyone else out there that doesn’t get more fit no matter how much they train

I’ve trained in martial arts for 20 odd years, exercised just as long yet am nowhere near as strong as I should be. In fact Im weaker than I used to be and especially weak lately. I’ve been running again for months after slacking during the winter but I haven’t not only not improved but gotten worse. I couldn’t even run a mile today my endurance is so bad. We should improve over time but instead Im far weaker than I used to be. Is there something wrong Im doing or am I just old now?