Research now shows that there is truth to the saying: No pain, no gain. This recent study posted by The New York Times revealed that if you don't exert your body just a little beyond it's comfort zone, you won't see much results. In order to realize the greatest benefits from your workouts, you need to push yourself.
Catecholamines are released only during exercise that the body perceives as stressful. In other words, when the body is in Fight or Flight mode, you release a different set of neurotransmitters that effect your overall wellness than if you weren't exerting your body a little harder than what it's used to. Without some physical strain, you will still see muscular adaptations, but if your exercise is light and induces no catecholamine release, but the changes may not be as pronounced or complete as they otherwise could have been.
As with any other physical changes you plan to take on, be sure to discuss them with your healthcare practitioner before starting.