You'll be fine in row 48's pair of seats so long as the specific aircraft assigned to your flight isn't N219UA, N220UA, N221UA, or N222UA. Those are the 4 77Os that have 48ABC and 48JKL.

In the United app, these 77O planes will appear as "Boeing 777-200 | #2xxx", with #2xxx being one of #2019, #2020, #2021, or #2822. You will see the plane assigned in the Flight Status area of the app starting around 4 days prior to departure. But even with the aircraft assigned at 4 days prior, the seat map won't switch to the exact layout of the assigned plane until 2 days prior.

All that said... aircraft assignments can change unexpectedly at any time. There is an element of luck that accompanies seat-selection strategy.