It is usually much easier to convince an office worker than a manual labourer that they need to get regular exercise, after all if you have been climbing up ladders, digging threnches, laying pavers and loading/ unloading vehicles all day you shouldn’t need regular physical exercise should you?, Wrong!.
The way you use your body when you do most jobs is very different to how you use your body when you exercise. When you work you are focused on the task at hand, if you are right handed you will use it to operate power tools for instance because it is more productive and safer than using your left hand, so a right handed worker will usually have much more muscular development in their right hand, arm, shoulder and upper back than on their left side, this right/left muscular strength imbalance can give you a crooked spine and cause premature wear and tear on that side of your body. When you exercise, the body is used in a much more ambidextrous way which keeps your spine straighter by fostering even right/left sided muscular strength.
Work can help make you physically stronger in a task specific way but it is unlikely to do so in a way that will give you good stature and proportion. As far as stretching exercise is concerned how many occupations require you to actually stretch your muscles in the course of executing your duties?, unless you are a Yoga teacher (for instance) leading the class by demonstrating exercises, who ever stretches at work?
Watch a person who is exercising with good technique, the way they lift weights is much different to how a worker might unload bricks from his ute, exercise is as much about technique as it is about effort, in order to exercise properly you must be posture conscious and aware that you give all your major muscle groups a good even workout, the focus of physical effort at work is about achieving tasks so you get paid, the whole mission is different in focus and execution.
If you have a manual occupation and feel too physically tired after work to exercise you may need to do it before work and make a point of exercising on the weekend too, a strategy to accomodate exercise no matter what your circumstances is important and will have a direct consequence on your health in the medium and long term future. If you are a resourceful person you can make opportunities to exercise, you don’t need a state of the art gym to exercise, you can use 5 minutes in your morning tea break to stretch your legs or do some neck stretches sitting in your car waiting for the lights to change, you will probably find many inventive ways to exercise if you go looking for them.
Work and exercise are different activities with different purposes and objectives and should be viewed as such, labour is no substitute for exercise.