First, read this- our Archbishop here in Baltimore, the former bishop of the Military Archdioces, has urged priests to get themselves in shape. Here's a key quote from one priest, who is cited as a great example to other priests:
"I looked in the mirror and asked myself, 'Do you want to be a fat old man before your time?'" he said. "So I looked at my lifestyle, began to study nutrition, began to run ... and then diversified my workout program. It's been a big part of my priesthood. You're more aware of yourself, and the gift of life. When we let our bodies go, we really in a sense misuse or abuse the gift that God gave us."
Take the orthodox priest in the picture, for example. His story is over at the Functional Hand Strength site, but the mere fact that he is pressing a freakishly huge kettlebell should be an inspiration to other priests. It is possible, nay helpful, to be into hardcore fitness as a priest.
In Catholic theology, masculinity is innate to the priesthood (though not the other way around). Having said that, priests should not let themselves go just because they have forgone marriage. After all, fitness wasn't ever about being attractive in the first place. What was it about then, you ask?
"The Glory of God is man full alive," St. Irenaus said. God wants us to live fully; to use all of our faculties and skills for the glory of God. This, however, might bring up another quesiton- God is God. His glory comes from Him being the eternal, all powerful being and cause of all existence. How can we, almost nothing in the vast expanse of the cosmos, add to His glory? Well, in one sense we can't- God is indeed that amazing and unfathomable Being, no matter how well or badly we live. He doesn't need us. We are, however, made by Him because He loves us and has made us in His own image. We are meant to reflect His glory. By living up to to human vocation - the holiness and fullness that God calls all of us to - we can reflect his glory better; alternately, by living apart from him, wasting our lives in laziness, gluttony and other sins, we choose to not reflect his glory well.
Therefore, by living up to what God calls us to be as men: strong (spiritually, mentally and physically), full of self control in every sense, continuously striving for virtue and to walk with Him, we glorify God. In short, one way we can glorify God is through fitness. Fitness, moreover, helps us in seeking holiness. It combats laziness, teaches self-denial and self-control, which in turn combats lustfulness and gluttony. Pope John Paul II once spoke of sports as a "school of virtue," precisely because of the way sports/fitness can form your character for the better.
We cannot, of course, note that fitness helps us on the path of holiness, without noting how important holiness is to fitness. Of course, you could be strong as superman and be a terrible person- that is granted. So how could holiness be important to fitness? Because it places it in a meaningful context. It gives more reason than simply looking better or simply being able to lift large amounts of weight. Someday our good looks will fade; our strength will weaken when we get old. We will all die someday. If all you are seeking is something temporary, then what is the point?
Some see this as a reason for hopelessness, and a reason to forgo any such vain effort as exercise. This is the cynic's camp: "eat drink and be merry, for tomorrow we die...." But that is not the mind of the Christian man. Christ came that we "may have life, and have it to the fullest!" That does not just apply to the obvious eternal life that we seek and hope for; it also applies to the here and now! "Be perfect," Christ said, "as your Father in Heaven is perfect." We are called to perfect ourselves continuously, in large ways and small. Fitness in and of itself is a tiny form of perfection next to holiness. But as a way to seek virtue, to learn humility by confronting your weakness face to face, to glorify God both in our souls and in our bodies, fitness fits perfectly within the wider realm of holiness.
In this way of thinking, the perfection of our bodies and that of our souls become bound together, and that is as it should be! God made us body and soul. Not a body with a soul, or a soul with a body. We are our bodies and our souls. With this in mind, dive on in- glorify God in your body and soul!