I have to qualify anything I say with the fact that I have scant knowledge of training or exercise in general. What I do might not be right for you because of differing age and circumstances. I am seventy-one, have never attended a gym, and have devised routines for myself using trial and error over several decades.
The main principle, as I see it, is to gradually establish a permanent habit of daily physical activity of varying type. Like any change of habit, the process requires discipline, in much the same way as working on music needs discipline, and in my experience takes about three months, after which it tends to carry on under its own momentum.
My present specifics are easy to list but may not be right for you. I exercycle, quite hard on reasonably high resistance for an hour every day. I do resistance work for about an hour every second or third day using expanders and bullworkers. I walk a great deal, usually rapidly and at least a few miles a day, but not to any structured plan. I labour in the garden, mowing, weeding, digging, sawing, just general maintenance, but done rather quickly, usually for an hour a day but it does depend on the weather. The figure of eighteen hours is just what my Fitbit tells me the weekly total of physical activity has usually been, not necessarily all structured training; perhaps I used the wrong word.
It is the initial formation of the habit which proves hard for many people, a bit like stopping smoking or drinking. However, after the struggle, a curious mental reversal takes place, and it becomes hard not to exercise because you enjoy the effects so much, and discipline and pleasure become one state. Also, don’t forget that rest, sleep and diet are part of it too. No good training your guts out then eating a load of rubbish and staying up half the night boozing. Common sense is required in building things up gradually. If you start with a hiss and a roar you will probably pull a muscle or tendon and have to stop. A lttle and often is much better than an all out strain once a week.