Race Across America champ Lon Haldeman says to spend an hour a week in that extreme breathless state, where you cannot even talk, in order to see real significant gains in fitness (short video 3:26):

https://www.youtube.com/watch?v=9FfA6VM7iT4

The fact that your legs felt fine is maybe an indication to use a higher gear when you start approaching your max on climbs. I was always taught, "Legs burn, use a lower gear, lungs burn, use a higher gear."