In Americans Spending Less Time in Nature a biologist at University of Illinois releases trends from data gathered on outdoor activities. He found that over the last decade, the percentage of Americans that fish, camp, visit parks, or take part in other nature-based activities has declined at a little over 1% each year (With the exception of hunting which has stayed the same, and backpacking which has risen slightly).
While people who do participate in those outdoor activities may say 'Good, more peace and quiet for me out there', it can have an affect on the future of our natural spaces. People who do not visit the parks or other natural places might be less likely to protect or support them. Without funding and support, our public natural spaces may cease to be.
So what are you waiting for? There are probably many natural areas near you that you don't even realize are there. Check out your state's DNR website and explore the possibilities.
Maybe you would really love to do more outdoor activities, but don't have anyone to do them with and don't want to go alone. I was feeling the same way, until I met someone who loves the outdoors as much if not more than I do, who was also member of a local camping/hiking group through Meetup.com. This is a great place to find people in your area with your interests.
Now you have no more excuses (cold doesn't count either, see Winter Camping), so Get Out There!
FYI- The USDA Forest Service's national campaign “Get Outdoors” (launched this February) is trying to encourage children to explore our national forests, learn about healthy outdoor exercise, and learn more about the natural world.