It's the stress injuries from overdoing activities in your twenties and thirties that catch up with you when you hit your 50's, 60's and 70's (which is where most of us posting on this thread are). Keeping fit helps but won't prevent stress injuries. Hiking and backpacking are rather unique exercise in that about the only specific preparation exercise is hiking and backpacking. Sure, you can hike up and down stairs or around your neighborhood with a pack, but that's not the same thing as up and down steep mountains on uneven trails. It's best to start easy and work up!