I think its a great thread, and brings up alot of issues & choices. ANY self defense classes have their worth; as has been stated, if for nothing else than cardio. Even some of the advanced stuff is good; pressure points & joint locks. The downside of it is the "art" part of it, IMHO-you have to go through a lot of useless stuff to weed out what is applicable. Hence, I suggest a more combat oriented system, which can be difficult to find. They strip everything down to what works, and thats it.
To touch on the 3 second rule; one thing we had done in the past is to try punching, full force, for 1 minute. You will see how tired you get-anything less than a full on hit likely isnt going to bother your attacker. If you are fighting for more than a minute, you are either losing, or about to. Without the cardio, its tough to go toe to toe with someone for even a few mninutes. Never mind one at full power.