You can't get a seat with leg room if there is no seat with leg room to have. Which is a great shame.
Let the airline know when you book the flight, check-in (try checking in online early so you can choose yr seat) and speak nicely t' th' cabin crew when you get on.
But every frequent flyer is trying to get the comfiest seat and we all try to think of some angle so that we can get it.