When I first rode clipless, I too had the m520's. After multiple instances of being able to unclip easily (or at all), I lowered the tension of the spring. You should see a hole towards the rear of the pedal, with a + and - on either side. Try turning to the - side, that should loosen the spring and make it easier to disengage. Start off in very tiny increments until you get it right. I'd take it to the flat area you mention and test and adjust there. It doesn't necessarily matter on the foot you don't put down at the stopped position, but you can match the detensioning so that they are equally easy to disenage out of. As you get more comfortable with riding clipless, you can always turn up the tension if you find yourself unclipping unintentionally.