In honor of the fact that I just put on my wetsuit and went swimming in the Atlantic (water temperature 60 degrees), I figured I would write about the single most beneficial workout I have come across.
Swimming is truly a full body workout. Your upper body will grow muscle mass and tone simultaneously, your legs will strengthen and your stomach will flatten. Oh, and did I mention it's great cardio and burns tons of calories. I am not going to spend a ton of time explaining the benefits - I am sure you have seen a picture of Michael Phelps somewhere.
There are a few clear problems to swimming - 1) access to water, 2) it's boring, 3) fear / ability. I'll address them one at a time.
1. Access to water can be very tough. The easiest is a swimming pool obviously. Join a gym with a pool, but you'll have to pay a premium. And don't expect to find a lane at peak times. Make sure you have a schedule flexible enough to get in the pool. If you live on the coast, swim in the ocean. Find a bay or otherwise protected area where the waves won't crush you. I like swimming so that I breath under the arm closest to the shore - that way I don't suck in a huge wave. Midwesterners can find a lake. But those are all of the options. If you can't think of any water to swim in, you can probably just stop reading this.
2. It is pretty boring. You can't talk to anyone or watch tv or even really look around too much. Having a workout partner doesn't help too much in the pool for entertainment. Pickup an underwater iPod case and some headphones - they exist and they work.
3. A lot of people can't complete a swimming workout because they are too anxious the whole time. Anxiety in the pool stems from not being able to breath when you want to. Start off swimming slowly, taking a breath during every other stroke (for instance every time you stroke with your right arm) and constantly thinking about your breathing. It takes a bit of practice but once you can breath while swimming as you breath while walking, it will be much more enjoyable.
Now get on there. Your goal is be able to swim 1500 meters. That is a bit less than 1 mile. Your first workout can be 200 meters and super slow. Within a few months you should be doing 1500 meters in less than 30 minutes. I'll let you plan out how to build up to that.
The best my body has ever looked was when I was training for a triathlon. And my back has never felt as good as when I was swimming a lot. Swimming is seriously a cure all - easy on the back, knees, ankles, etc. and an amazing workout.