I do push-ups every other day, and want to do them every day - most of the time in sets of 25, depending on how many I want to do for the day - I like to do at least 200 total. Sometimes I start out with a set of 50 or 60 (always with my favorite music blaring!)if I do them later in the day. Chin-ups I do about 3 times a week, starting with a set of 10, then several sets of 5's, 4's, and 3's. I want to work up to my biggest set at 15 or more. I've done 100 push-ups in a row, and I want to do 125. This "mission" started when I was 55.
Lower body - I do something 5 days a week using med balls, stability ball, Gliders and body weight. Lots of lunges, plies, variations of squats.
I got into it by accident - I was training for a push-ups contest at work last year, and about the same time decided I wanted to learn how to do chin-ups. I was amazed at how quickly my upper body got defined, which I could never accomplish after years of lifting weights. Lower body, too - I got smaller and tighter, especially in the inner thigh area. My quads got more defined, too - another one of my problem areas.
Let's face it - ballet dancers and people in the military (like right out of boot camp) have the best bodies. What do they have in common? Body weight exercises and lots of cardio! That's good enough for me!
Another thing - take the work "can't" out of your vocabulary! If a 55 year old chick can do 100 consecutive push-ups, anything is possible in the realm of exercise!
"It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt." Mark Twain ;-)