Although this isn't GAAP, its the closest we have.
The executive summary is that the US Government lost $1,047 billion in FY 2016 and its net financial position (assets minus liabilities) was a negative $19,292 billion.
How much interest was paid in FY 2016? It's hard to find in the report, because you have to use the right search words, which are: "Interest on Treasury Securities Held by the Public". The total was $273 billion.
So, no worries. The debt will never have to be paid back, and the only real cost is the interest, which is totally manageable.