So, when I retired, I had an aim to "get back into shape". Good luck with that. And of course initially I did next to nothing anyway. In 2016, I went back to a baseball fantasy camp, something I did in 1992. I had a blast, but clearly needed to be in better condition, so I started, and man it's tough.
These days I do something every day, if the weather permits I'm outside running, doing sprints, whatever, and then spend some time in the gym. Today it was just gym time, and my muscle stuff is improving a lot. Flexibility is tougher. I have lost 20 pounds, I had really ballooned to almost 240. Obviously that extra fat as enemy number one. I hit 98.0 kg yesterday.
I went to the wife's pilates instructor last week and go a few ideas for flexibility.
My advice, if you're 40, and tired, is to MAKE TIME to do a little bit every day, and more when you can, even if it's a nightly set of pushups.