Plenty of exercise, regular visits to a doctor, and a healthy diet can lead to a healthy body.  It's pretty much true. But there is a better benefit from Internal Martial Arts. The reason is because the body gets worked, yes. But so does the mind and spirit. This is because when you are practicing martial arts, you are concentrating (should be), your breath is following the body, the body is following the intent, and this is harmonized with the mind or consciousness (which is monitoring the body movement). With this kind of concentration and movement,(done correctly) there is a kind of communion between the practitioner and the Universe. This 'connection' is the same as meditating. The movements of the internal martial arts (or Qigong) open the meridians in the body to allow chi Qi (chi) to circulate freely between the organs and the rest of the body. If taught correctly by an experienced teacher, every time you practice a posture or form, you are pretty much effecting the body as if you were receiving acupuncture to unblock blockages. After several years one will have cultivated more Qi, Jing, and Shen-and will have extended their lifespan. On the other hand, if you practice incorrectly, there can be adverse affects and/or just an aerobic benefit. 
    Walking  keeps your metabolism up but that's pretty much all it does without proper breathing techniques 'movements' as in the walking of Bagua Zhang.  Jogging jars the knee joints and back vertibre affecting your later years. Sports are good when you are young (and meridians are still mostly open and the muscles/joints are still undamaged) but usually come with a coach yelling at you to you what he can't even do himself, an opponent trying to beat you, and the crowd of onlookers cheering during games. You find yourself being pressured to do good physically by the coach, your team members, or your very own ego (Money, Fame, Glory, etc). This eventually leads to an injury, resentment, and or loss of normal movement. (Sometimes all at the same time.) For what? Money? Fun? A College Scollarship?  I guess it can be fun when we're young and talented, but when you blow out that Knee or Back, it's all over! At least you can watch your favorite team make it to the playoffs, but still end up with nothing but memories in the end...
   Nevertheless, when one reaches middle age, your muscles weaken, aurthurittis sets in, and disease begins to apear. Nowadays these problems seem to be happening even earlier due to the way the world is. (Pollution, Global warming, War, Stress, Junk food, and of course, the side effects of Legal & Illegal drugs and alcohol.)  It's ironic that the rate of Cancer, Diabetes, and various other diseases are running rampant more today, that let's say 100 years ago (When there was little or no treatments for them.)
   Although most "EXTERNAL" Martial Arts are good for a healthy body, theyl still fall into the category of general boxing ability with aerobic benefits that will only decrease with age. But if that's what you want, go for it...