I've not been there yet, thank God! My only thoughts would be that your Doctor wants you to be mildly active, aerobiclly that is, on a consistant basis. I wouldn't overdo it, just maintain a moderate elevated heartrate for 40 minutes as he mentioned. You may ask him about it first and not us.....its pretty hard to overdo walking unless you start running! Most everyone with good legs can walk but I'm with you, I prefer to ride, although I enjoy a walk every now and then to work different muscles and stay stretched.
Be smart, be consistant, be moderate, until you get the green light to up the intensity.