It is age.
I used to be able to take classes, walk 10 miles, etc, etc...
My tortured journey started 4yrs ago. I was exercising fine, walking fine, etc...and then we moved. I did not exercise for close to 2yrs on a reg. basis.
Well one day I got my Jillian Michael's tape out and injured my back doing lunges. And there is where my rabbit hole started.
You are just never the "same" after an injury. It takes total dedication to get yourself back up to speed.
It has been 4yrs since my initial injury and I am finally on the right track.