I understand where you are coming from. There's nothing stopping you from adding miles when you feel like it. Having an infant probably means you won't much.

Start looking at bike trailers (would make a great baby shower gift) so when the baby is old enough you can take them for rides and supplement your exercise that way. A bike trailer was one of our best child related purchases. A Baby Bjorn and a Kelty child carrying backpack were right up there too. Those are things that let you be active while carrying the little one.

If your wife likes to get her exercise in, give her time to do that (by taking care of the baby) and she'll likely reciprocate. Actually, whether she exercises or not, she'll benefit from some time to herself. Make sure your doing your share and she'll not object to your getting some time too.