Ah, the ease with which one can fall out of shape. Physical fitness, you eludeth me! (I just made up eludeth.) (I think.)
When you're out of shape, it's so easy to be out of shape.
But! And I can say this out of personal experience that once you work out for a while and see some gains that further motivate you then you can get into a real routine and you can actually think, "I can't believe I haven't always done this, it's so easy to do it." And being in shape really truly does feel very good.
And now, after around eight months hiatus (and, therefore, after losing over a years worth of cardio and 12 weeks of consistent weightlifting gains) I try again. Summer is a great time for it.