The 45 degrees is the curve down on the y-axis, with time being the x-axis, i.e. steady decline.
In answer to your years up time, it seems up 8 hours at 10 years, 6 at 20, now 3-4 hours.
Now average 12 hours in bed (8-10 sleep; 2-4 awake)' and the rest piddling, reading on sofa. Some away time each week, maybe 3-4 hours out to eat. And I'm in relatively good shape.
I was in super shape when hit by a 'bug' that started the whole decline, having to drop each exercise sequentially by year 3, i.e. jogging, cycling, lifting, swimming. (No booze, no smoke, having had to give them up earlier after college.). (BTW, I was super stressed running my own company at initial onset - after 2 week's vacation in Lake Tahoe+, fwiw!)
I hope you'll have success changing your lifestyle to sedentary now versus getting on the grind. The grind will win every time
, and you will otherwise feel like the very devil is on your back!
You could try one of Erik Johnson's books*, a forerunner in this whole current mess, about my age. Try Amazon for some paperbacks or download.
Good luck.
Book: "
Back from the Edge", with Lisa Petrison, PhD, Kindle/Amazon