The chivalric writers spent a lot of time talking about chivalric acts. Ramon Lull speaks of the duty of a knight to defend his worldly lord:
The office of a knight is to maintain and defend the worldly lord, for a king who has no barons has no power to maintain righteousness in his men without aid and help. (Lull p. 29)
Likewise, Geoffroi de Charny writes of the simple virtue of doing things well:
Therefore, you should know for certain that there is no one who can or should excuse himself from performing well according to his station, some in relation to arms, others in relation to their clerical vocation, others in relation to the affairs of the world. (Charny p. 63)
None of them, however, really address physical exercise as a chivalric act. It's not, to their way of thinking: keeping one's body in condition to fight, or to labor in the fields, or to labor in the service of God is not a chivalric act, it's merely a part of chivalry. It's assumed.
Lull does say:
Knights out to take coursers to joust and to go to tourneys, hold an open table, to hunt harts, bears and other wild beasts, for in doing these things the knights exercise themselves to arms and thus maintain the order of knighthood. It is wickedness to love the customs to which a knight practices but to despise the order to which he belongs. (Lull p. 30)
But it's not a subject that the chivalric writers explore in any great depth.
Not directly, at least. Let's examine Charny's words again. In order to perform well in according to one's station, one's body should be in condition to be able to do so. This doesn't always mean keeping yourself in tip-top shape so you can climb a ladder to rescue a child from a burning building, or chase an armed robber down a dark alley and wrestle him to the ground. It also means staying strong enough to help unload a truck full of canned goods for a women's shelter. It means having the agility and endurance to spend a day building a house for a family in Louisiana. At the very least, it means having the ability to do your work well, to care for yourself so that others don't have to.
It's also an issue of respect. That body you have is the body the Maker (by whatever name(s) you may wish to use) gave you. Barring fairly serious scientific advancements it's the only one you're going to get. It behooves you, then, to do what you can to care for it.
For me at least, exercise can be a meditative thing as well. The discipline of the weights, the tug and stretch of muscles. Letting the bar down gently so the weights don't clink; pushing my breath out as I push the bar back up. The good pain of pushing myself past what I think I can lift; the sweat bursting out all over my skin and the sweet smell of it.
And the treadmill, which I used to hate, but have since fallen in love with. I'll still stroll around the block, still wander the woods but the treadmill lets me focus on the walking. I can't slow down to watch a bird or laugh at a prairie dog; all I can do is set one foot in front of the other, again and again and again until the rhythm of my feet is all there is in the world. The symphony of moving my body frees my mind and I float away, lost in deeper contemplation than I manage when I'm home, body folded into the proper meditation position but the monkey mind jumping and screeching.
It's not just about losing weight until your BMI matches whatever the mass media says it should be; it's about living your life in the good way. If you bring nothing else away from this posting, please, at the very least, take away the thought of getting outside and taking a walk.