Do I have to hit 30 minutes?

Here's my situation - I have 20 minutes here - 15 minutes there etc.  I am doing the elliptical for 20 minutes a day, which for my weight and intensity of 3.0-3.5, burns over 400 calories at a time.  On most day's I just dont have theextra time to go for a full 30-45 minute workout (not all on the elliptical) in a row.  My question is do I have to hit 30 minutes all at once or can I do 20 in the morning on the elliptical and 15-20 in the afternoon walking?  I can always make 20 minutes - I can' always make 30-45 minutes at once.

This sounds bad but right now I'm really looking for a way to get the most calories burned in the shortest amount of time and I'm not really concerned with the other great benefits of exercise so is the 20 minutes / 400 caloires enough?  Later on once I lose 50-75 pounds then I'll be wanting to add on things and tone, shape and strenght train.

Just do what you can and don't stress over it. I've heard experts say 30 minutes of cardio in one shot is the same as 3 - 10 minute sessions. You burn the same amount of calories if you're exercising at the same intensity. If that's your main concern, number of calories burned.

Sounds good to me! As Earl says, "experts" say thirty minutes broken up into small sessions counts, so there ya go. To me, it is FAR more important that you establish the habit now anyway.