I'd just be as completely honest as you can with your friends. Tell them exactely what you told us, if they really are your friends then they'll understand and respect your space.

I know that between my schooling and a girl now and then and my dad working the only time we really get to be together is in the woods or on the water. I cherish every minute of it, your kid will too. Start them young and they'll learn to love it. I sure did.