Hi m~nien,
My dad is 68 and we work out regularly together. He does 25 minutes on the treadmill, 15 minutes on a lifecycle and about 45 minutes of weight training (nautilus machines). Now of course he wasn't able to do this when he first started but he's getting stronger all the time and is like a teenager in his enthusiasm. It sounds to me like you are also doing really well. I'm 40 and twenty minutes on an elliptical nearly does me in!
I started a running program a few weeks ago and for the first week or two I thought my body was going to give out. My legs, hips, shins and ankles were killing me between runs but eventually all of the pain went away. Today I ran 2.25 miles in the morning and 1.25 miles in the evening and I can honestly say that I'm totally pain free as I type this. The body simply adjusts over time and things become easier.
My adivce would simply be to keep moving each day as the body allows. I promise that it will get easier and the soreness will cease to be a problem. God knew what he was doing when he created us and as the article says.....the body is begging to MOVE.
Happy exercising!
Dave

M~nien - obviously at 70, your body does not have the recovery abilities of a 30 year old, so just take that into consideration. If, after calisthenics training you're very sore, the next day do something different, either calistenics that work different muscles, or a different type of exercise, such as walking, or swimming, (insert exercise of your choice here). There is no reason not to exercise frequently or with intensity when you're older, as long as you pay attention to your body!!! and know your body's limitations. You'll need more rest/recovery than a younger person, you won't be able to take the same abuse (such as high impact sports, things which would give me at 22, sore muscles, some aches and bruises would be likely to break an older person's bones, or pull muscles and rip some tendons) Take your time to increase intensity or resistance, pay close attention to what's happening (rather than blundering into a new activity), and give yourself proper nutrition...oh! and stretch

take care!
Meg