My go-tos are: "Not too bad." and 'I can't complain.'
At least where I was raised, it wasn't 'manly' to respond cheerfully or positively, and the person questioning you absolutely did not have any interest in your well-being and it would be awkward to describe how you were feeling and why you felt that way.
It's also possible to to shoot a 'Hows it going?' right back at them and cease interaction immediately.