Yes. As a fellow “creative” who makes their living from writing, I’ve decided to spend some time each day writing for clients, and some time writing for myself. To take your gym analogy, it’s like spending some time on the weights, then doing some cardio, and rounding things out with stretching.
The cardio and stretching might not fill out my bank account, but they make me more balanced, which means I can write with more poise and happiness. As freelancers, we put so much pressure on ourselves to perform, that sometimes we forget the magic and beauty in the creative act itself.