One of my favorite topics to think about while running. I find most of my running centres around 2/2, for anything hard. When I run at MP or therabouts, is when I experiment. The norm for me is 3/3 here... gradually becoming 2/2 as I get a bit tired. I have tried all the obvious variances, but my favorite 'odd' one is 2/4. I like this one in the middle period of an AT run. I sharply inhale on the 2, and on the exhale (4) I let it out slow through pursed lips. It's a good 'precursor' to the inevitable 2/2, when 3/3 is not adequate enough anymore, if you know what I mean