To answer the title question, you work out your body. That specifics of that question highly depends on your current situation. Like age, current fitness level, what you are training for (strength, cardio, weight loss, etc), and what you have access to.
Highly recommend Jellyfin on your NAS. Sounds like that is what your looking for. Very straight forward and easy to implement compared to other self host options.
Essentially, vid files located on your nas, and then any device on your wifi can stream the vids.
If your looking for your own personal netflix, jellyfin is your answer.