In other words, don't worry about your future in the industry. You're doing fine.
Out of the normal 45 to 60 hours I work a week when I am doing design work, at least 75% of that time is spent doing high level thinking. I am not an anomaly in my firm.
I don't accept nor do I respect the idea that a person is only good for a couple of hours of high quality work to their employer per day.
On the other hand, I think people who say that they are only able to get through 4 hours of work in a day, really specifically mean programming. And it's probably true in that limited sense, but all the other stuff - communicating with others so you know what program to write, helping others so they can work more effectively, is actually very valuable. It's just a common programmer failing to not really see that stuff as 'real work'. It's even worse as programmers get promotions and are expected to spend a significant amount of their time on management type activities.
Sometimes people claim they can be productive for 8 or more hours per day, for weeks on end. But likely what's happening is they are tricking themselves into feeling (and looking) productive, while their actual productivity finds an equilibrium far below what it could be if they paced themselves.
If I had it my way, I'd work half time programming and half time working a public facing job.