After draining the coolant, putting straight distilled water through the system, running the engine for about 10min, draining it again, put more distilled water in, running the engine, draining it again etc. etc. I did this a few times over and now when draining, the water comes out pretty darn clear.
Which brings me to my question...how much distilled water is STILL IN THE ENGINE that won't drain out? I need to take this into account when I measure out the new coolant and distilled water mixture, so that I end up with a 50/50 mixture.