I think you need to find some form of exercise that makes you
want to get out there and do it...there´s a guy who´s written a lot about
flow, that really put into words what I feel when I have a great work out:
Mihály CsÃ*kszentmihályi - Wikipedia, the free encyclopedia
Reading up on it might help you get a better idea what you "should" be looking for...
I find it running, cycling, swimming, spinning as well as playing stuff like badminton, squash, soccer, basketball and loads of other things...my advice would be to look for it and not stop until you find it...
It´s weird too, when jogging I need to do about 10k, to really
feel it, but when I do, it´s like catherine was saying...a good rush...I´m an addict
I also think it helps on having one "challenge" for each workout, whether that´s about really pushing it up-hill, doing 5 more reps/lbs in the gym or whatever, have
something that challenges you and motivates you to keep reaching beyond that comfort zone that most people find after a few weeks...
Of course it also helps to have some semi-longterm goal...like a race or just a day every month or so when you
max and see the progress you´ve made...have something to measure yourself against so that you see progress...it could be your weight, waiste-line, heart-rate, or whatever...
...also scheduling your exercise...write it down, if it´s on paper/in your calendar, it becomes "real"...at least to me...
just some ideas...