I do fitness walking to keep my hip joints movable and non-aching.
When you fitness walk properly, the idea is not to lunge forward as you take each step, but rather the goal is to keep your back foot on the ground as long as possible before you bring it forward to take each new step. This stretches the leg back past the straight-with-the-back angle, like the guy hanging his leg down off the table in the stretching video. When I walk properly for 3 or 4 miles, I feel wonderfully afterwards and my hip joint areas feel really good.
I have found that biking won't do this at all, and if I only bike for fitness and ignore my walks, then after a couple of weeks my hips start feeling creaky and aching again.
Even when your leg in on the downstroke and your knee is almost straight while pedaling, your hip joint is still bent as though you are sitting, so it doesn't get your leg back to keep the hips fully flexible.
just my own experience.