Ian has a clearly buff build even in Chapter 1. He just doesn't have his muscles outlined like some bodybuilders but he has the mass in his muscles, thus has a healthy diet and works out constantly. If he wasn't in good shape, he'd just be either skinny like Danny or have a bit of a belly like Perry or even both, skinny torso with a belly. Good shape isn't measured by combat skills, only how you look. I somehow doubt that movie stars with chiseled bodies all know martial arts and can defend themselves in a fight against most people without proper combat / martial arts training.