If you can set goals for others you are in a good position to set goals for yourself because you understand the basic principles of exercise physiology. My walking program does not involve the use of a treadmill. I am a Nature scientist, therefore, everything that I do is done out in Nature. The walking trail that I walk is a very hostile one, that taxes one's endurance, sometimes to "the critical point." The critical point is, where I consider there is a risk of injury to one's body, physically, mentally, or spiritually. We want to stop before we reach this point that I consider critical. That point is, of course, different for different people. As a personal trainer, I know that you are following me in my thinking about this.
Therefore, ask yourself, "Where is my critical point?" I cannot answer this question for you because I do not know you well enough. I am assuming that you are in excellent condition, physically, and maybe, also, mentally, however, I have not even the slightest idea where you are spiritually.
I wish you the best.