Fitness mobile app which emulate training with a coach

Hi guys.
I always love being trained by a coach and listen to his (her) voice during exercise, counting, motivation words, swear words.

The idea is simple: Fitness Mobile App which emulates training with a coach + training log and diary.
It will have simple physical exercise programs which you can do home or anywhere.
For example, 10 push up + 20 squads + 10 jumps (Sorry I am not a fitness trainer). But training will be followed by coach voice recorded specifically for with training set.

For instance, you make 20 squads, coach voice count all the time, and at the end voice tells you: "Oooh boy, you are the second Arnold Schwarzenegger (read with an accent)"
or if you did less: "I see (stretching), someone gets sick today, (short pause)... (little aggressive) go and make +5 more" :)

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My questions are:

1. How does it sound? Would you use it?
2. How can I find out if this idea worth it? I mean in business words. What should I do to analyze market to understand that this app need anyone expect me? Is it possible to do or I need to create app and see results?
3. Do you use fitness mobile app? What do you miss? What do you like?