First, when both the man and woman exercise they both have something they want to achieve. Whether it be to lose a couple pounds or to gain strength the couple has something that they can help the other complete. Not only is there a common goal but there is someone right next to you cheering you on to be the best that you can be. It also creates a fun pursuit that each of you can challenge the other in. For example, my wife has always been able to run for long distances at a very good pace. I am more of a weightlifter and have never been one to run for miles on end. So, between the two of us we challenge one another to reach out and try the other's interest. Now both my wife and I run and lift weights. We rarely have the opportunity to work out or run together, but the overall goal is to help each other be in the best health possible, and what better way than to grow in health together. We men are meant to 'tend and keep the garden' given to us by God and our wives are meant to help us be the best spiritual and physical heads of our households, aside from a prayerful and sacramental marriage, exercise is one of the best things to help each other reach their God-given roles.
Secondly, our vows require us to look after our spouse's health. "In sickness and in health" was not suggesting that whether you want to be lazy or active or whether you want to eat pork rinds or salads is fine by your spouse. Being a child of God hands on the responsibility of taking care of our own health and when you said 'I do' you promised to keep your spouse in his or her best health as well. There is always a balance to be had when it comes to our diets and our activity levels, within right reason a person can stay in shape and keep their spouse in shape, decently easy. With a little help from the theological virtues you can both grow together in mind, body and soul. Fortitude will help when you want to be lazy. Temperance will help when you want to eat fast food every day. Justice will help a couple give each other the love and respect due to a creature of God. Prudence will help a couple find the right plan to achieve each other's goals.
Thirdly, I don't think any man or woman wants their partner to lose the physical attraction that originally brought them together. Men want their wives to feel safe when they are together and want them to know that no other person could harm her as long as she has her man. "His arms are rods of gold...His legs are pillars of alabaster..." - Song of Songs 5:14-15. Women want to feel attractive, they want their husbands eyes to light up when he sees her and to feel that he can always brag about his beautiful wife. "Oh Noble Daughter, your curving thighs like jewels, the product of skilled hands." - Song of Songs 7:2. The poetry of the Song of Songs is the poetry of the theology of the body, the beauty that we all contain should be something we are proud of and therefore should be kept and cared for as the prize that it is. Physical attraction is part of our human nature, we are called to look beautiful and handsome, but we are not called to put it out on display for all to see. A marriage is meant to allow a man and a woman to give all that they have to each other, especially their bodies, wouldn't you want to look the best for your spouse?