Captain America is one of the most, if not the most, renowned battle tacticians in the Marvel Universe. If this fight arises he will make the factors of the battle play in his favor. He knows what Tony's weaknesses are and he will play on those throughout the battle (such as Tony's massive ego and constant over-confidence). He could easily feign weakness and loss and while Tony would be monologing about why Cap shouldn't have picked this fight, Rogers could go for the kill (or win depending on the scenario).
Another scenario is where Cap disables Tony's armor (which has happened several times before in the comics) in which case you have an enhanced human with enhanced speed, strength, agility and a healing factor against a man stuck in a limp piece of metal. We all know the outcome here.
Some things the Captain has going for him in short are:
-His healing factor
-His ability to dodge bullets at point blank range (as demonstrated in the comics)
-His unbreakable shield that can redirect lasers
-His fist (which shatters bones with one punch)
-His immense strength (bench press 1000+ pounds depending on source)
-His peed (50+ mph running)
Whatever the case may be, Rogers is smart, and won't fight when he knows he might lose. He makes people fight on his terms, and in nearly every battle he makes sure he has people to back him up if things go bad.
You see, iron man or tony stark is one of the brightest minds in marvel. Cap isn't really known for his techno based knowledge. Tony has many machines that can beat Cap. He also has many allies. If you want an example of one of Tony's many machines, well, there is the Hulk buster.
You known, the one that beats hulk.