I agree with many of the above - whether or not you get the bike, start a walking routine. Walking is probably the single most efficient exercise there is, especially for the beginner. Start slow, start with a comfortable pace and increase speed as you feel yourself getting in better shape. Persistence is the absolute key - exercise in some form every single day. And remember, also as mentioned above, diet is essential. The general rule is that fitness and health is 70% diet and 30% exercise. Or put another way - you can’t out-exercise a bad diet.
Yes! Although some folks have back or joint problems that make extra walking an ordeal, it's great if you can do it. This is what makes dogs especially helpful -- you can ignore a bike or avoid a walk, but your dog will motivate you to get out there every day!