I understand your skepticism.
But I absolutely believe we are all capable of far more than we think.
The mind is lazy. Or maybe a better way to phrase it would be to say that the mind seeks efficiency. We try to get the most done with the least amount of effort.
There are many examples of people achieving amazing things once they have decided in their minds that they won't quit. I will say that it's far easier said than done. But I think the potential to tap into that is within everyone.