My first ride ride was six miles on a coaster bike. When I got home and got off the bike, my legs were shaking so bad I had a hard time walking back to the house! I drove a minivan then and the next day, I could barely lift my leg up high enough to get in. I suppose you could say I overdid it.
Anyway, I agree with the previous posters, don't worry about miles, just get out there and pedal but NEVER FORGET: The number of miles you've ridden OUT, is the same number you're going to need to PEDAL BACK, so don't get carried away. Good luck and have fun!