"To pay off the debt, you're going to have to give up some living standards," said Copeland. "You may have to stop traveling or going out for dinner."
Some people will relocate to a place where the cost of living is cheaper. You can compare what your expenses would be in different locations with the "Elder Index."
If you have enough equity in your house to sell it, you will, in certain cases, be able to avoid a number of costs because you're no longer responsible for property maintenance.
"They may be able to move into a retirement community, where there may be a better social aspect than living in a house in the suburbs with a bunch of young people," Copeland said. "Or they may have to move in with a relative or friend, to share living expenses."
More from Personal Finance:
Protect your bank accounts from rising debit card fraud.
The Bank of Mom and Dad is open for a quarter of working millennials.
Putting bitcoin in your IRA can sink your retirement.
But if you're underwater on a mortgage, he said, "it doesn't make any sense to sell your house, because then you're just left with a debt and no place to live." People in those circumstances might consider renting out a floor or room in their house, although this requires a lot of work.
Another way to bring in more money, of course, is to work part-time if possible.
"We encourage people to work longer, if they can, because every year you work is one year less that you have to finance in retirement," said AARP's Trawinski.