"As long as a person exercises and exerts himself a lot, illness will not come upon him and his strength will increase."
"Exercise is a form of activity involving bodily movements that may be strong or gentle or a combination of the two and which cause changes in the person's breathing, which becomes more rapid." Rabbi Moshe ben Maimon, Maimonides, The Rambam (1135-1204).
There we have it from one of the foremost and respected rabbis and scholars in Judaism. Oh, and it's a commandment: The sages considered it so important that they equated it with the commandment to remember the giving of the Torah/Law: On the verse "Guard yourself and greatly guard your soul lest you forget…the day that you stood before Hashem your G-d at [Sinai]" (Deuteronomy 4:9-10), the rabbis explained "guard yourself" means preserving one’s health.
I shouldn't have to say more, than to get off of your butts. Just in case though, a little trip through history...
Judea, occupied by the Roman army, and along comes a band of rebels circa 66-73 C.E. These men worked in fields, in shops, with animals etc, almost as Rambam's time would have known work to be, and they had to be in good shape. In other words, they exercised. These men were so hardy that it took approximately 80,000 of the best soldiers in the world at the time to put the rebellion down.
A little further back we go and we come to the famous story of the Maccabees, that led a revolt against another of the worlds best armies, and gave us the story of Chanu, Hanukk, Hann... okay, the holyday that nobody and everybody can spell (Almost). Do you think that they were able to do that by wishful thinking or that the Syrian army killed itself?
Turn the clock back a little more and we find the icon of Judaism, David. Shepherd, musician, poet, guerrilla fighter and soldier, and later, King of Judea. But, to get there he had to fight, and conduct a series of hit and run missions against the king and his army, and after that, fight the Philistines, Moabites, and the Syrians (Yes, them again). He and his men had to have to get off their asses and move...
----I'm going to skip over the Book of Judges just due to the fact that the entire book is full of ass kickery and bloodshed (The original story of the "300" is with Gideon FYI **)... because the people had to.... Exercise!
Even the story of the exodus from Egypt, and the collective beginning of the Jewish history gives over a lot of speculation that the men were soldiers or at least mercenaries before they even left Egypt. Exodus 13:18 "The Children of Israel were well armed..." and later they were organized into soldier formations. They fought Amalek, and later, under the leadership of Joshua ben Nun, they took Jericho, Ai, and so forth and so on...
If these people lived in a time of hard physical labor and exercised to stay in the kind of shape that they knew was required of a fighter, how much more so is it important in an era when our daily "activity" is largely stationary or sitting, this includes office workers sitting behind a computer, drivers and students in the schools, including the Yeshivot (Religious schools). Compounding the the problem is today's average daily diet, which is loaded with carbohydrates, grains, and processed sugars. Inadequate physical activity and unhealthy diet are at the root of many of the health problems and illnesses with which people today are afflicted.
Ask yourself or your doctor, if getting off your ass is right for you?
**The argument could be made that the original 300 was with Abram (A.K.A. Avraham), but he had 318 troops. Close, but no falafel.