Is it normal that i dont work so hard anymore?

I used to be a hard worker. I was very proud of my ability to get hard jobs done competently and efficiently. I could work very efficiently whether the task at hand took strength and perseverance or focus, or both. I had an uncle tell his friends that he thought i could accomplish anything. I had three different employers tell me that i had done the hardest days work they had ever seen. They were all over 60.

But im 45 now. I dont have the focus or strength and i certainly dont have the drive. I find myself daydreaming and avoiding the harder jobs. I dont perform tasks with immeasurable pride as i once did. I feel the aches in my back, shoulder and knee constantly.

Have i become complacent? Did i become one of those lazy people that i once had no time for? Am i burned out for work because i used it up all i had years ago? Is it just my age and other people go through this too ( though others my age seem to do better than me )? Is it the pain i feel in my body?

Please let me know if you have an opinion or theory. Thanks.