Re: Another about to start thread...
Janet's got the right of it -- there's really a wide range from dojo to dojo as far as how strenuous and body-stressful aikido training is, so you do have to go visit dojos to get a sense of this. Observe a class and talk with the sensei about your physical issues, to see if they can be accommodated.
Honestly, I don't think that aikido or any martial art is the best way to get in shape. They tend to require fitness more than promote fitness, if you know what I mean. That's not to say that someone who's out of shape can't start martial arts training, but you do need to be a bit careful, and also realize that it's not the most efficient way to get into shape. I think it would also be a good idea to supplement your martial arts training with other activities that will promote fitness and better prepare you for your time on the mat. For someone who's been pretty much totally sedentary, I would recommend starting with about 45 minutes to an hour of moderate to brisk walking followed by 15 minutes of stretching, each day that you're not on the mat. Don't walk so fast that you feel pain or real strain, but do walk fast enough to get a "glow" going, and learn to stretch properly. After a week or so, increase your walking time and intensity slightly. Keep at it, and up the intensity or difficulty (by throwing in a hill) or duration every time it starts feeling easy. If you do this regularly, it will help your body to recover from your aikido training, and at the same time you'll be gradually building up a base of aerobic fitness. Eventually you'll probably want to move on to another activity like biking or running to get more of a workout, but do it as a gradual progression. Just keep at it, and in a few months you'll look back and be amazed at how far you've come.