Don't worry so much about mileage at first. Pick a route that you think you would enjoy and start there. You can worry about miles later.

How old is the kiddo? Those baby seat things worry me. Could you get a trailer? That way you could also use the bike for errands.

like Grog and somebody else said (Nike, maybe? I don't remember) JUST DO IT.

as for your diet, well you already know what you're doing wrong there. The only person who can help you with that is our friend Will Power not to be confused with Max Power which is the alias Homer Simpson got from a hair dryer.