Linsy, if 20 mins is daunting, and I totally GET that, two ways to go.
Either find a way to pass the time, like get an audio book (library has playaways, which are portable players with a book preloaded, they loan for free just like a regular book, you just need a pair of ear phones..our library sells them for $1 each..easy!), and you ONLY get to listen to it when you're walking, for example. Then set a time each day, EVERY day, rain or shine (walk inside your house if you have to..no, really!), and stick to it. I find it easiest to work out in the morning, that way it's done, and no matter what happens the rest of the day, I did that right. If I wait, it's much harder to break out of "real life" mode and take time for me. If you don't like audio books, then you could listen to a favorite radio show, or music, learn a second language, whatever appeals, but just something to distract your mind and make that time more fun. 20 mins will pass quickly with a good book playing in the headphones.
Second approach would be to start REALLY small. Like, walk to the mailbox or the curb and back, or set a timer for 3 minutes, or put on a song and walk for just the length of ONE song, then done! Excercise? Check! Pat on the back.
Do that every day for as long as it takes for that to become a habit, easy, nothin' to it, then add to the time just a little, one more song, go for 5 minutes instead of 3, or walk to the corner and back instead of just to the curb.
You can do this with exercise dvds, also. Instead of doing the whole thing or nothing, do just the warm-up or the first 5 minutes, whatever seems so ridiculously easy, you can't think of an excuse not to do it. Leave it in the player, and set it up the night before, after you wake, go to the bathroom, turn the coffee maker on, go to the tv, do the warm-up, turn it off, pat on the back, then have your coffee and get on with your day. Voila! After a week or two of that, I bet it seems just as easy to do just the first few minutes of the "workout" portion, do that for a week, and so on.
The first time I ever had any success with daily exercise, I did 15 crunches, every morning, BEFORE I got out of bed. Just woke up, did 15 crunches in my jammies, that was it. When other "good intentions" fell by the wayside, I kept that up, because it was just too easy not to. I didn't need shoes or a water bottle or anything, and it felt good to do at least that much, every day, first thing, no matter what. Eventually, I dropped it because other workouts were in place, but that was my first success, so if you can find something like that, whether it's crunches, a short walking session, or 5 minutes of a dvd before you'll let yourself have coffee, just find something and do that.
No, it won't burn 500 calories, but it will help you start to feel good about yourself, your belief in your ability to keep your word, give you success at establishing a routine, and even a few minutes will start to build a little strength. I got really good at crunches, let me tell you.
So, there are two ideas on how you could start. You could do one, or both. You could take a 20 minute walk with an audio book (you could probably do an audio book with your WATP dvd muted, too), and do 15 reps of some ab exercise, or push-ups against a wall. The biggest thing is just to start with whatever you can handle, emotionally, not just physically. If it seems too hard, if it's "all or nothing", often we just end up doing nothing, but if you can just start, then it gets easier to keep going, a little longer, a little faster, as you get more fit and start feeling good about it. You could even go do something, right now, 3 minutes, and be done for the day. Hint-hint..