Exercise is Medicine: Make a Habit & Get Your MED.

Being physically active is preventative medicine

Exercise is medicine.
I think at some level we all know this. We've all heard it in some shape or form, whether it was from the doctor or some new research thats making the news. But really we already know it, at some intellectual or cognitive level at least.
Do we put that knowledge into action?
Probably not! Sure, it's easy to tell ourselves that we're too tired, don't have time, don't know how, are too unfit to go to a gym, don't enjoy exercise, don't know the best exercise for (insert whatever ails you here), don't want to exercise on our own or whatever the reason excuse is. The truth is its just a matter of habits.
Warning, tangent ahead!!

We are built to be energy efficient, right down to the way we think.

I like to use the analogy of walking across an overgrown patch of waste ground: The first time is always the most challenging, because you're pushing through the weeds, picking your way through thorns and carefully doing your best not to come a cropper on obstacles unseen in the growth. When you get to the other side and look back theres a bit of a path left behind. If you take the same route again soon, its a bit easier because there's still a bit of a path; some of the weeds will have sprung back, and need to be pushed aside again or pruned back, but it's still a bit easier than the first time. With regular, repeated travel, you create a path which you can follow easily; the grass is tramped down, the thorns pushed back and the obstacles clearly visible.  However, if you don't go that way for a while, it'll be either partially or fully overgrown and you'll be starting again.
In neuroscience (the study of the brain) there's an expression for this..

Neurons that fire together, wire together.

Neurons are the nerve cells that make up our brains and internal communication systems.Every action, every thought depends on a message making it down a path. When a path is taken more frequently, the body improves that path by making it wider, so the message can reach its destination more quickly and efficiently.

The thicker bands have more neurons, allowing better transmission of whatever message they carry.

Over time, less used paths get overgrown, In the brain, the path less travelled thins, as energy is needed to maintain each and every pathway. The thicker the pathway, the more energy needed to maintain it. This ability of the brain to rewire itself in response to the demands placed on it is known as neuroplasticity.

The point of this basic introduction to neuroscience?

Same old habit or a  new direction?

When we are in the HABIT of saying "I can't" we thicken the connections on that path, making this the answer we default to. How many times have you caught yourself saying"I can't" to something you've never even attempted? How do you know since you've never attempted it??? The answer is you don't know, you're just defaulting to the most energy efficient answer. It's not necessarily the CORRECT answer, its just the easiest one for your brain to throw out there. Now that you're aware of this, you can take action to stop it from sabotaging your success. Anytime you catch yourself saying that horrible four letter word, pause and ask if that is the correct answer, or is it the just the easiest one to reach.

What's the next step?

Ban "can't" and replace with "can try"

Change your language. Eliminate that four letter word (can't) from your vocabulary. There are other good reasons to do this, but they'll keep for another day (if you really want to know more on this ask me about brain regions activated by language and neurotransmitter cascades in response to certain words). Get into the habit of saying "I can" and then giving it a go. You'll be surprised at what you can accomplish when you let go of this horrible four letter word! Obviously, a single day of saying "I can do it" will not overcome years of a habitual negative, but it'll start you along a new path. Of course, when you're forging a new path, there will be thorns and pricks that have to be pushed aside, there will be obstacles over which you may stumble, but pick yourself up and refocus on reaching the destination. Over time you'll become aware of what you stumble on most often and will eventually catch yourself before you trip up, and you'll take a slight detour around  it.

Back on topic.
So, exercise is medicine, and the main reason most people don't stick to an exercise plan is mostly a matter of changing habits. It's easier to stay motivated to keep working on making those changes if you are seeing results from the actions you're already taking. Exercise is medicine, and like any other kind of exercise, it has a minimum effective dose. If the medication says take 200mg then you take 200mg. Why? Because less than that may not be an effective dose.

For exercise, regardless of the desired physical training effect, the minimum effective dose is challenging yourself. 

The training session or workout is only a stimulus that can create in your body a chain of events that allow you to become fitter, slimmer, stronger etc. This is called adaptation. If the stimulus isn't enough of a challenge, then no adaptation or change is need for the body to be ready for that level of challenge the next time it faces it.

If it doesn't challenge you, it doesn't change you.

Of course, in order to make the most of the stimulus, appropriate nutrition and rest (including sleep!!)  are needed. What foods and how much of them will depend on  your current situation and what it is you're aiming to achieve.