Whenever I fly United on US internals, I upgrade to "economy plus" which gives a bit more leg room and usually for about $30.00.
On a 777 flight from IAD - DEN that put me in a business class seating area with bigger seats but on a 320 from DEN - LGA I just got an economy seat with six inches more legroom.
Don't know if it's available on longer haul but it gets me a bit more space without paying loadsamoney for a load of extra food and drink I don't really want.
I quite like United .. the cabin and the flight attendants remind me of my days going to school on London busses.