I know exactly what you mean. When I exercise, I feel sooo much better - physically mentally and spiritually, yet when I'm feeling down, am stressed and busy, it's the last thing I want to do when even though, I know on a conscious, logical level that it will do me the world of good.
Just 30 mins of working up a good sweat will make everything feel better, and you'll also feel like you've achieved something
I find the hardest part is usually getting changed into my exercise clothes - it's like putting on a whole new mind set. So pull out the trackies and the runners...
Good luck, Nay - get out there and get that blood pumping!

Just keep moving! And don't be lazy...
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SW: 74.3kg - 1/1/09
CW: 71.1kg - 3/5/09
GW 62kg