In what ways does your body improve when you make a habit of running a few miles every day?
Note first of all that if you want to run, it’s actually better to run every other day than every single day. Running strengthens certain muscles, but if you run the same way every day, you’ll be training exactly the same muscles, and in the same way. It’s better to have a more varied schedule, to cross-train and to give yourself some time off.
Still, it’s better than nothing. It burns calories and increases cardiovascular fitness. As your muscles get acclimated to the effort, they will become stronger and have more endurance. You will find that you run the same distance in less time, and you should increase the distance to keep running for the same amount of time. Estimates vary, but the general advice is that you should exercise for at least 20 to 30 minutes at a time in order to derive real benefit from it.
There’s definitely an advantage to doing something every single day, as it does become habit, which encourages you to keep doing it and to make time for it. That’s a mental change, not a physical one, but the mental benefits of running are at least as big as the physical ones.
Physically, running also appears to improve the overall strength of your bones and your joints. Bones respond to the stress of running by becoming denser and stronger. It is a myth that running damages joints; joint damage is caused by a lot of things as you get older, and running is seen in most studies to help the joints.