Babies are time-consuming. They demand a lot of attention, and unfortunately as a long-distance runner, your training also requires a lot of time and attention. But there are ways your baby can help you train.
Okay, this is the most obvious but if you're running 100 miles, you are probably going to be running through the night and into the next day. Having a new baby means you're probably getting only a few hours of sleep each night. Just think how much less tired you will be on a lonely trail at 3:00 AM, when your body is already adapted to being up at that time!
This doesn't really come along until your child is mobile, but you'll find that you'll need to move quickly to keep her from rolling off the bed, crawling up stairs, crawling down stairs, playing with the only electrical outlet in your house not baby-proofed or chewing on a poinsettia leaf. You'll need to accelerate quickly, only to have a brief rest period before you need to stop him from doing something else potentially fatal. This is a great way to build speed, possibly taking seconds off of your marathon time!
To get the most training out of your child rearing experience, I recommend using cloth diapers. When running a fifty miler it can be tough to keep your energy gels down. Dehydration and fatigue can conspire to make your stomach want to eject those precious nutrients your body needs to get you through a race. But if you're scraping baby poop out of clothe diaper every day your gag reflex will slowly desensitized and you'll find that even a greasy cheeseburger on top of two quarts of cytomax will go down smooth.
(You don't want to end up like AJW):
I know I'm only scratching the surface here, so if any of you ultra-running veterans have any other suggestions, please comment!