Almost 40 year old here. Here's the thing: motivation is overrated. It comes and goes, and if you're planning on relying on motivation as the main driver of progress, you'll fail. You just need to get it done. How? Just use the old cliche "How do you eat an elephant? One bite at a time." Specifically: get off the couch, go to your closet, take off your clothes, put on a pair of shorts and a t shirt. Now go put on your sneakers. Yes I know, you're tired and this sucks, but look, you're already in gear. Only took two minutes. Now go outside. And now just start running. Literally every single step you take makes you better off than you were sitting on the couch. Want to go the gym route? Same path: Collect gym gear, get in car, drive to gym, get changed, go to weight room and just do something. Literally anything is better than nothing. You can literally start right now: google how to do a burpee, then get off your ass and try to do 20 and see how long it takes you. Tomorrow, aim for 25.
If you really need motivation though, then try this: You're 40. How many years do you think you still have left before it all goes downhill and you can't reverse it? 5, maybe 10? 15 if you're lucky? Brother, you're already late to the game. You won't max out your natural muscle potential, you're too late for that. Every single day you waste now is bringing you one step closer to being some shitty 50 year old who can't climb the stairs in his own house without getting winded. Wanna keep being fat? Well, in about ten years you'll be looking at heart disease, diabetis or cancer, so that should be a fun ride for you.
Fuck motivation. Just go do something, anything other than sitting on the couch and hold yourself accountable.