We have all felt that way at times - i was for the 1st 2 wks of dieting but the past 2 days i've exercised so eventually you will get that up and go - and another thing to think about is just how good you will look once you are all slim and toned and healthy looking!
Just get out and do it! Dont give yourself time to think about it. Im not the biggest fan of exercise so I just set a time and I just go. After awhile you will get use to it and feel guilty for NOT going.