Someone said it above, but it bears repeating: if you want to truly be motivated, try to start an exercise regimen with someone else. There will be days when you just don't feel like working out, but since you're accountable to someone else, you'll suck it up (and trust me, you'll feel better for it). And it works the other way, they'll want to wuss out, and you'll hold them accountable. That is definitely an excellent motivator.
I've found that just a quick (~20 minutes or so) jog with some push-ups, crunches, and pull ups makes for a near-perfect workout for me. 3 days a week (MWF) at about 30ish minutes is enough to get in my exercise and not feel like I'm devoting all my time to it. Works out great for me.
Oh, before I forget, the other thing to keep in mind is that if you haven't worked out in a long time, you're going to have a harder time getting "up to speed" as it were. Just push through it, and don't get discouraged. Right at the beginning is the easiest time to lose your motivation.