Unfortunately, most people I hear that try to keep Cornish X broiler chickens as pets die around a year old. Rarely they live to be two years old. I have been told that it is a very sad, miserable life for them to live and all of the ones I see people raising for the fair lay around and refuse to move. However, mine are six weeks old right now, still fly extremely short distances (about 5-6" off the ground and for about one foot), jump up to eat on a their feeding platform (it is just 5" above the ground) and love to run and play. Heck, they still love to play with the baby turkeys through their enclosures. So, I am not sure if the difference is how you raise them on how lazy they are, but but it is food for thought.
Another idea that is food for thought, is to keep an eye on them. If they become where they do start to look miserable and it is getting entirely too hard for them to move around, it may be more humane to butcher them or have them put down. I always say to treat all your animals alike. My poultry and fish are treated just as good as our family dogs or kitties and given the same amount of attention. Heck, I stand by this so much, even my fish are taught simple tricks (like "giving kisses" or "dancing"). It really is just some food for thought so you choose the most humane decisions for them, whether that be trying to restrict their food and prolonging their lives as much as you can, or experimenting with them to see if exercise keeps them healthier, or something else.