charlesatlas.com

If you are just starting out, or are over 50, you have to begin slowly and work up. Those over 50, should realize that you can't do quite as much as years ago, and adjust accordingly. Those starting out should begin slowly and then work up to doing more gradually, to avoid injury and burnout. Henry, you have been working out for years, and appear to be in good shape. Others, your age, might not be in such good shape. I am 70, and I can still do 60 plus pushups in one set, 100 situps and deep knee bends in one set, and walk 3 miles daily. I feel great, and am in great shape for my age, but I can't do nearly as much as when I was younger, and I adjust accordingly. I used to jog 10 miles a day, and do hundreds of pushups, situps, etc. daily. Now, I do just one set of each exercise until I am tired. Yes, I am in good shape, but not even close to the shape I was in 30 years ago.

I'm joining this discussion late, but it is my understanding that Atlas does not call for twice a day. Its three times a day. Morning DTs, afternoon (or anytime) walk or run, and evening DTs. All three are necessary as they keep the metabolism going at a high rate.

I'm joining this discussion late, also, just to add that according to my understanding, Atlas calls for four times a day.

DT in the morning.
Stretching before launch (lesson 12)
3 miles walk or run
DT in the night

And as Atlas says, many of DT exercises can be performed during the day at odd moments.