By the way, do you know why 21 days are chosen? After the experiments of scientists, it was proved that it was during such a time that a person remembers actions and this becomes a habit in him! Well, oh well, about training: my advice is to do complex exercises for all groups of the main muscles, with a different dosage of tension, of course, there must definitely be strength exercises, for example, push-ups from 1 to 21. A good example of squats. It’s worth adding a run if you usually don’t use it. A good option would also be to choose a distance and increase it every day. By the way, you need to prepare for a run, immediately explain to your clients that there are various shoes for classes, that is, for example, https://perfectshoesforyou.com/best-running-shoes-for-bad-knees/, which can help them if they have any problems. Most people, unfortunately, do not know about it, but it is important, it will improve their life!