I think your understanding of national debt is incorrect. Much of the debt isn't like a mortgage, with a schedule of payments to be made that include both principle and interest so that after X years the debt is eliminated. Instead, much of the debt is interest-only, so payments are required (and built into the budget) but no matter how much we repay as interest, the principle remains. What you probably want to ask is "how much of an annual surplus would we need to pay down the debt in X years?"

I think your understanding of national debt is incorrect. Much of the debt isn't like a mortgage, with a schedule of payments to be made that include both principal and interest so that after X years the debt is eliminated. Instead, much of the debt is interest-only, so payments are required (and built into the budget) but no matter how much we repay as interest, the principal remains. What you probably want to ask is "how much of an annual surplus would we need to pay down the debt in X years?"