how do you pay back a debt by talking out more debt.
You don't. OTOH, all the fiat dollars in existence were created by deficit spending & fractional reserve lending, while interest paid is compensation to wealth holders for dilution of value of dollars they hold.
The US govt doesn't really need to borrow, at all. It's a holdover from the Gold Standard & a sop to the Wealthy. The govt can acquire whatever funds are necessary through taxes & seignorage, the printing of coins. The Treasury could make a $1T platinum coin the size of a dime, deposit it with the FRB, and write checks against it in a perfectly legal fashion.
The Treasury can also create a collapsing option with the FRB. They could sell the option for $1T, or any amount, an option against future earning, and the FRB could sell it back at a later date for a penny.
The fact that Dems even entertain Repubs' bullshit wrt the Debt Ceiling shows that they're serious & mindful about it all in a very conventional way, that they want real answers. That can't be said for Repubs, who just preen & posture on their so-called "principles" while offering nothing concrete at all. They're the party of no ideas, just empty bluster-
http://krugman.blogs.nytimes.com/2012/12/13/party-of-no-ideas/