Sex is a good workout. Now, I just have to figure out something to get my right arm buffed up.
Seriously, just start small. If you start with full cross-fit classes five days a week, you'll burn out before you even start. Begin with short walks. Get out and get some air each day. After a week or so, you'll find you have a little more energy. Increase the walks, and slowly work your way up to hitting the gym or some classes.
There's also, seriously, a bike-powered laptop charger. Use that, and you'll get some exercise in while you work on your computer.