Overfilled oil tank, drained transmission fluid

Overfilled oil tank, drained transmission fluid. What do?

I had decided it may be a good idea to learn how my car works, so I really know what I'm driving, rather than just driving. I decided to go with the first thing anyone could think of: an oil change. I went out, found the proper oil, proper filter, etc. However, after getting all the supplies, instead of locating the drain for the oil, I found the one for the transmission fluid. I drained about 4 quarts of the stuff out (still thinking it was oil) and then proceeded to replace the filter, add new oil, etc. By the time I had realized what I had done, it was too late. The transmission fluid previously in the car had already been disposed of. For a while, I had thought I had overfilled the oil. So, I did what I had researched, and drained about another quart out of what I still thought was the oil. By the time I had realized that the fluid was too red to be oil, it was too late. So, basically my car is dead where it stands until further notice. I've recently located the ATF dipstick (what I presume to be it) and the transmission fill cap. Likewise, I've now located the real oil drain. As of currently, I have a mix of conventional and synthetic oils in my engine. About 8 quarts or so. The Mazda is a 2.0 liter engine, and only requires 4.5 quarts of oil. So, it's double the normal amount. Should the oil be completely drained and the filter replaced? Likewise, how do I go about replacing the transmission fluid? Or, would it just be a better idea to take it to the shop and have them do their thing?

In the words of Gob from Arrested Development: "I've made a huge mistake."