The long slow cardio is probably the best IMHO.
+1 to this. Of course, there are many facets to "fitness"--muscular strength, muscular endurance, cardio endurance, flexibility, etc., and you ignore any one of them at your own risk, but if I had to pick one aspect to emphasize, it would be cardio. Your metabolism becomes more efficient at burning fats, so you can go longer without "hitting the wall" or feeling those hypoglycemic shakes during a long hike. In a survival situation with little food, this means you function better without feeling awful. I feel that you can withstand stress better, whether chronic or the sudden adrenaline dump, when your cardio system is fit. Of course, there are the long term benefits of blood pressure and cholestrol control, etc.
I do think "wind sprints" or intervals, or whatever you want to call them do have a valuable place, though. Well, there are variations. Going full-out, anaerobic, my-legs-are-burning wind sprints is hard on the body. However, doing something similar but only going to upper range of your aerobic capacity for a time then back down repeatedly also has many benefits that you won't easily get by just going slow all the time. For me, a pulse of about 160 is just at that threshold when I can feel myself getting a bit anaerobic--the breathing gets a bit ragged, my mind suddenly feels busier, and the exercise isn't so easy anymore. If you pay attention, it gets easy to notice when you hit that point without taking your pulse. Unless you regularly push that upper limit a bit, I personally find that it's easy for your conditioning to actually slip over time because your body gets too used to performing only at that lower level, no matter how many hours you can keep up that lower pace.
Don't overdo it, though. I see lot's of people who seem to be training at that upper, or even anaerobic, point all the time. They're the ones who leave the cardio machines looking like someone just hosed them down. I often only see them at the gym for a while and then they disappear. You just can't keep up that kind of workout day in and day out. Even if you can do it physically, it is mentally tiring, too. Like I mentioned, your mind gets rather stressed at that intensity level. It's not really a fun or peaceful place to be. Slow and easy is the base, but periodically pushing the envelope a bit is good, too.
I recently started taking a yoga class for an old back injury, too. It's great! I wish I had tried yoga a long time ago. It's not a miracle cure or anything, but it has worked in ways that "normal" stretching never helped me with. It can also be surprisingly tough. I was so sore every week after class for more than a month before I got strong enough that I could actually feel relaxed while doing the poses. If you're interested in yoga, I suggest taking some classes. It's important to get the biomechanics of the poses correct to prevent injuries and stretch the correct things, and that is hard to evaluate yourself. It really helps to have an experienced instructor guiding and correcting you until you get the basics correct.