Well, driving 6 miles with a grossly-overfilled crankcase is not good, but it was probably less injurious than having driven for…probably…well over 1,000 miles with insufficient oil in the crankcase.
Lesson #1: The “oil light” is most likely an oil pressure light, not an oil level light. When there is insufficient oil pressure, damage to engine bearings, cylinder walls, etc is the result. Unfortunately, you cannot turn the clock back to undo the damage that resulted from driving with a low oil level, but you can learn from this experience, hence…
Lesson #2: The dipstick should be checked fairly frequently on any car, and as a car ages it should be checked more frequently. Apparently, you are not in the habit of checking the oil level between oil changes, and you have to correct this type of negligence. Now that your car is clearly consuming oil, you need to check that dipstick at least once a week. If you continue to rely on a warning light, you will probably wind up destroying an engine that has already sustained damage.
Lesson #3: The part of the engine that holds the oil supply is the crankcase. It is not “a tank” in the same sense that your gas tank is a tank. You can run your gas tank fairly low without problem, but an engine needs to have its oil supply as close to the full level as possible at all times, and with a crankcase that holds as little oil as yours does, you can really do a lot of damage by running the engine with only about half of its normal oil supply. The object of checking the dipstick frequently is to never let the oil level fall more than 1 qt below the full mark. Personally, I replenish the crankcase as soon as the oil level has dropped by 1/2 qt, and my crankcase holds 7 qts!
Lesson #4: When adding oil, add it a little bit at a time, wait for a minute or so for the oil to drip down to the crankcase, and check the level. Do this several times, adding no more than 1/3 of a qt each time so that you don’t overfill the crankcase. Overfilling is bad because it causes the liquid oil to be churned into a foamy mass, and that foamy mass doesn’t lubricate properly.