The same principles apply whether you're a 20 year-old, 40 year-old, or a 60 year-old. Exercise is always good. The only difference between the age groups is the amount of intensity you should allow yourself.
I'm 40 now, but ever since 30 I've been concerned about being fit when I get to age 60-80. The thought of shuffling everywhere instead of dancing or being able to lift a box of books is terrifying.
Terrifying, and rightfully so. But you've obviously understood that you have some work to do in order to overcome this, and that's a pretty damn crucial first step.
I was in sedentary job, not exercising, and realised I wasn't going to get fit when I was 60; I'd have to get fit now, in order to stay fit as I age. I'm not sure what made me think of this, but is seems common sense.
It's good that you realize this intuitively. Getting fit becomes gradually more difficult with age. So does staying fit, but that difficulty curve isn't as steep. Maintaining is always easiler than developing. This goes for all phases of exercise.
I've managed to get into an exercise habit that just about moves me from the "not fit" to the "fit" category, but I'm hardly going to wow anyone with my fitness levels.
Doing something is always infinitely better than doing nothing.
On this site, I consistently try to make one tremendously important point; walking is exercise! There's so much to be said for an exercise where you're simply moving, but keeping your back straight, chin up, and your heartrate slightly elevated. If you're able to do this for even 15 minutes per day, you're well on your way.
I like to recommend 30 minutes, with the extended caveat that it shouldn't be a part of your work day. For instance, walking back and forth between office cubicles is NOT exercise. But taking a walk outside, in a variety of uphills, downhills and flats, is amazing for you. And it also help with other aspects of your fitness, for instance mentally, but allowing you to clear your mind, get some fresh air, and it can almost be meditative if you'd like it to.
Are my concerns valid, and what types of exercise (at 40) will prepare me for an active lifestyle at 70?
Absolutely valid. And I have another point that I always dish out to anyone and everyone; for the love of all that is good, go swimming! I cannot emphasize this enough, so let me just say it again, GO SWIMMING!
Swimming is this miracle of an exercise where you do this repetetive, aerobic movement (cardio), but with the added resistance of water (strength training). And on top of this, there are so many styles and ways of swimming that allow you to work your entire body.
I consider myself a fairly decent swimmer, and I'm still learning new strokes and styles, and each time I try a new one, my body reaps the rewards, because of what we call "progressive overloading" where your body develops new neural patterns to adapt to new challenges. Not to mention that you're burning fat, and building muscle at the same time.
Going to the gym is never a bad idea either. Talk to someone who works there, and they will gladly help you set up a workout program designed for general fitness.
In short, a combination of walking, swimming, and a couple days per week at the gym, is HUGE. And even if you decide to do only one of these, you'll be doing yourself big favors. And hey, if you do decide to just do one of these, I'd highly suggest you pick swimming.