I'd actually say don't push yourself. Push yourself to take it out, but if after 15 minutes you decided you've had enough, turn around and go back home. Don't push yourself to the point of having frazzled nerves, because that will cause you to lose focus if you're panicking a little. I started slowly (200km in my first five weeks) but now the mileage has started racking up. At the two month mark, I was all of a sudden at 900km. If you got stressed, or spooked, that's when I'd say don't be afraid to head back home. Only do what you are comfortable doing - doesn't matter how long it takes you to learn, it is not a race.