Good on ya. A little flat spot is way better than the alternative. Some riders deliberately skid the rear for way longer than 10 feet for fun or to rotate it around quicker than a static friction circle might allow.
Other points:
1) If you locked your rear tire, the likelihood is that you didn't simply just apply the rear brake too hard but rather that you hit the rear brake too hard relative to the front brake. With sufficient weight transfer up front, the rear end will get light and will lock up under braking way earlier than normal. This is why ppl practice emergency braking until they are relatively comfy with it.
2) if you "slammed" your brakes, this tells me that the brake levers weren't covered (ie. fingers resting lightly on the levers). If the brakes were covered, then the tendency is to simply squeeze and modulation is part of the equation leading to more control. It won't be an on / off switch type of application. Many defensive riding advocates will suggest to cover brakes when entering a zone that is not 100% clear (ie. at an intersection with cars).