The closest thing we've seen to a true non-powered superhero is Kick-Ass -- a schmuck in a hood with no powers, training, money, or exceptional intellect. And so naturally, Kick-Ass gets his ass kicked. He probably should've named himself something like Free Ice Cream.
Shortly thereafter, after a battle gone horribly awry, he gets metal plates surgically inserted into his body. These plates essentially made him invulnerable to pain and punishment -- you know, like Superman? Even though he still wasn't a particularly good fighter, the fact that he could fight forever and never get hurt gave him a huge advantage over criminals who would eventually tire and punch themselves out.
And yes, by our logic, this makes Homer Simpson a superhero too. Absorbing endless punishment is a very useful tool in life, whether you use it to fight crime or make the local bartender a rich man.
The Black Widow is another example of this. Born in 1928, she should be long dead by now, as her only powers were peak athleticism, expert weaponry, mastery of every martial art imaginable, top-notch espionage skills, and having Scarlett Johansson's boobs. However, while still in her '20s, she was injected with a special serum that would render her immune to disease and aging, and allow her to heal at Wolverine levels. She's basically immortal now, in addition to everything else she already had. Besides the boobs.
Then you have the Red Arrow, who was already super-but-not-really, thanks to being an expert athlete, archer, marksman, fighter, and baker of souffles. But when he lost his arm in a fight, he was supposedly done. That is, until surgeons fitted him with a removable prosthetic arm that was -- you guessed it -- invulnerable to attacks. As long as he kept defending himself with that arm, nobody could hurt him. Not that anybody did anyway, thanks to the whole athlete-archer-fighter thing. After all, relying on just one form of invulnerability time and again could get boring after awhile.
It's hard to pick out examples for this one, because virtually every non-powered superhero in history can boast it. Batman, Punisher, Robin, Green Arrow, Hawkeye -- shit, even The Phantom -- have trained their bodies to be the absolute best body a body could ever hope to be. They're packed with muscle, but still stupendously athletic. They don't have an ounce of fat on them, but they aren't sickly skinny either, because that would be a flaw. Their stamina is just about endless, and they can function perfectly on little to no sleep for as long as they need.
Probably the only guy who isn't at peak physiblahblahyakyak is Tony Stark, who's an alcoholic who's slightly less shredded than his fellow unpowered-heroes, and who possesses very few physical skills to speak of. However, he more than makes up for it by owning and operating the Iron Man suit, which is basically a peak-physically-conditioned human in robotic armor form.
This is proof positive that, even if you spend every minute collapsed on a collapsed sofa, chugging booze you're too drunk to even taste anymore, you too can save the world from unspeakable atrocities! All you need to do is raise the trillions needed to build a giant suit of armor that nobody can hurt!
And if you can also master a hundred different ways to turn your opponent into pudding while you're at it, all the better.