If you want to visit the entire wall, I do not recommend walking. It is too long, and in some parts there is not much to see. The bike ride will take you between one hour and 90 minutes. If you get tired, you do not have to complete the circuit, you can return the bike at any of the Wall's main sights.
The terrain is quite flat , making the ride a nice pleasant experience. Avoid hot summer days at noon. If it is fall or spring, it is ok any time, but in summer stick to first thing in the morning or, even better, late in the evening.