What is a better way to exercise riding a bike or walking?

Exercise: Both are good ways to exercise. It depends a little as to your height, weight, condition of your joints, and your current physical health. I generally recommend to patients to start with a gentle exercise program, gradually building over time. For walking, find a comfortable route, or treadmill, proper shoes, start slow in time and speed. Gradually advance. Same with biking. Don't rush to be fit.