When I was paying on my house, I always sent extra money, and specified that it was for the principle, not interest. I made sure to write that on the payment voucher, my records, and on the back of the check.
When I was ready to pay off the house, it turned out they had applied all those checks to the interest! Well, luckily I had proof that it was supposed to have been applied to principle, and I raised cain and got them to redirect the money.
Anyway, the lesson is, if you possibly can, send extra money and make sure it is applied to the principle. You will be amazed at how quickly you can pay off a mortgage (or any loan) if you double up on payments, or at least send some extra money.
It feels GREAT to not be making monthly payments. (Not to mention that these days, with the economy being what it is and my business no doing well at all, I doubt I could even make the payments.)