2 bit speed has it exactly right. I made double payments on my mortgage every time that I had the cash to spare.
I had a terrible overtime event, 80 hours in one week is not fun, but paid principle for 5 months and the interest saved was much more than twice the extra pay.
Later, I got a low interest home improvement loan from the Credit Union, and paid off the balance. That loan was payroll deduction, and only 2 years, but paid off the last 5 years of the mortgage. Conversion to the Credit union loan ended the existence of the insurance and property tax escrow account that paid no interest, and we just wrote 2 checks a year. That money went into the CU and paid interest. We also paid the insurance 3 years at a time, for a 20% discount.
The money saved from those payments more than paid for my next 2 cars.