Newtons first law states that no force is required to maintain motion in a straight line at a constant speed. If newtons first law is true, why must we continue to pedal a bicycle on a level surface to keep moving?

You have to take into action all the other forces that affect the tires, one of them would be friction. As you input force into the pedals, the tires move, but the ground doesn't want to let the tires move, so friction occurs, when you jam on the breaks, the tire stops, and friction increases until the bike stops. Other forces are involved, and that's why we have to keep pedaling on a level surface.