An important advantage of cycling and walking compared to sports like football, running and squash, is the smaller chance of injuries. Therefore, you can always continue to build up your condition, without the risk that you have to stop because of an injury.
As far as that goes, cycling has an edge over walking, because you are less likely to injure yourself. On a bike, you make controlled movements and your muscles, tendons and joints are constantly and evenly loaded. When you walk, your bones, joints and tendons get a small impact with every step you take, which increases the risk of strain and inflammation. You also have to watch out for potholes and unevenness, because otherwise you could sprain your ankle. You don’t have to worry about that on the bike.