How at 46 years old, never having any credit, do I establish credit for a mortgage?

I have a 390,000 mortgage privately held at a rate of 8% for the next 54months then a balloon of approx. 300,000 to finish the terms of the loan. I may be able to cover the 300k before the payoff date. I would just be more comefortable with a longer term. currently my payments are almost 4k per mo. In order to satisfy by the end of my term I have to pay 8,675.00 per month to zero out at the end of my mortgage. you see the delima. The mortgage is on a 1.2- 1.7 m property (marina) that we put up almost 50% in cash on.