As you are probably aware the flight you were on gets further away from a diversion point than almost any other in the world so United will have some very robust procedures for what to do if even minor things go fail. LAX was still the nearest point, later it changes to Honolulu.
By the way ETOPS is not relevant here as you seem to have been on a 747.
The full emergency turnout with 30 fire engines is something that dismays many aviation professionals, as it unnerves the passengers considerably (as you). I am sure if there was any genuine concern the captain would have briefed you for an emergency evacuation on the runway. I know when push comes to shove the actual fire crews are always great guys but some of their procedures dreamt up by their backroom teams are way over the top and lacking in competent risk analysis.