This thread is insane, and demonstrates why people dislike MMA.
First of all, labelling the incident a "fight" or not is pointless. The poster of this thread was attacked, pure and simple. Not reacting to an attack with violence is an acceptable and noble response, and one that is not easy, as you may find yourself branded a coward because you refused to use violence. Of course, as the poster was physically attacked without provocation, counterattacking in this situation would have been justified as self defence.
Most of the "advice" presented so far is risky. Some of you are recommending fighting tactics (takedowns, ground-and-pound) that take years of specialized training to develop, and are utilized in a rule-bound professional sport. Mauy Thai, Brazillian Jiu-Jitsu, wrestling and the like are, at their core, SPORTS. That doesn't mean they have no use in self-defence, but they require a significant investment of time to develop properly, and they can be hobbled by the fact the recognize conflict as having rules. How does, for instance, Mauy Thai train to defend against an attack by someone wielding a club? They don't, obviously, because you don't use a club in a Mauy Thai match.
If you're interested in self-defence, look at the Wikipedia entry on Krav Maga
. Krav Maga is not a sport. It is a series of techniques soley designed to teach you to defend yourself if you're attacked. It acknowledges no rules in an attack. In your situation, Krav Maga would likely advise to intercept the blows with your left forearm and strike your opponent with an open, cupped right hand in the groin as hard as you possibly could. That will give you time to exit the area and avoid a further attack.
Of all the "advice" offered so far, I need to single out two pieces, one correct, and one incorrect. Correct - if you intend to actually engage in physical combat, work a heavy bag. If you don't know what it feels like to throw and land a punch, you will have no confidence behind your punching and they will be ineffective. When you do so, you must be properly equipped. Wrap your hands properly - or buy a pair of Everlast Evergel hand wraps, which do the same job, with the addition of really good wrist support. If you've ever missed a left hook and jammed your thumb on the bag, you'll appreciate the wrist support. Then get a pair of 16-ounce training gloves. Without proper equipment, repeated bag work with crush the small bones in your hand. The gloves also build your arms as you swing. It doesn't sound like much, but 16 ounces at the end of your hands is significant.
Bad advice - aim for the teeth. Aiming for the teeth is the kind of advice you get from armchair tacticians. All that's going to happen is you're going to split your knuckles down to the bone on the sharp edge of your assailant's teeth and bleed all over the place. Aim for the nose. A good, solid punch in the snotbox is both painful and disorienting. And for the love of the all-mighty, DO NOT headbutt without training. You're likley to strike with the bridge of your nose and break it.