How did you do feel when your at 192? If you were huffing and puffing and thought your lungs were going to explode, I would think you were near your max. You can get a test done to be scientifically accurate.
As is being discussed in another thread, The 220- your age is only an estimate. I'm 45, and I get still get a reading in 190's if the hill is steep enough and Long enough.
Zone training - it does work(from my view), but take some patience not to want to exceed the limits.