10 is a good number, and unfortunately it will hurt for a little while until the muscles are used to the exercise, but DO keep going. Eventually you will find that you will even be able to increase the amount you do, even if only little by little. However if you're in severe pain, maybe try doing them every other say until you get used to it.
Keep up the good work mate, you know the old saying "no pain, no gain".
PS: i do sit ups too and I HATE them, they're my least favorite exercise.