Oh come on now…
and you’re not even old 
There’s no point in training if you’re in your comfort zone all the time. Charlie said tempo should make you tired when doing it, it just doesn’t stress your CNS. There’s nothing wrong with getting tired, it actually makes you feel younger. And you say that you feel old, so maybe there’s a correlation here? I’m just saying.
I understand the thought behind training minimally while improving, but at least challenge yourself during that minimum time. :o