You might want to also consider how the day lays out. The times you teach might be easy or very hard. You have to work with your own body clock. I start my days at 7am with teaching Pilates and Barre Fusion. Later morning I ad the Aerial fitness classes and more Pilates and Barre..The rest of the day goes like that intil about 5 or 6pm. ( there are of course breaks inbetween classes and privates. I work 6 days a week and end of teaching about 25 to 40 hours of classes a week. (it depends on my other instructors availabilty on how much I work) I also certify instructors in Pegasus Barre Fusion Classes every few months. (This is my biz, so it's all I do. I put about 60-70 hours a week into it with all the admin and other stuff) I used to start some days at 6am, but I always felt a little sick to my stomach the rest of the day and way too tired that I had to stop. (I did it for 7 years and hated every day I did it) I also have a hard time witht the late night classes. I'm fried. Typically fitness hours our more like a split shift. I think it would be harder to have to teach only one class at a time then have a few hour break, then to put a couple all together. So consider your 'tooling up time" too. Getting there before students, preping for the class, picking music, driving, giving yourself enough time to eat, to change....My guess is that it would be easier to teach 3 shifts of the (10 classes) a week..verses teaching 6 shfits..because the more shifts the more prep you have.
Physically you will learn to pace yourself. I am not really getting a very good workout when I teach, because I am teaching, I have to also do my workouts on top of teaching. (Cardio, Pilates, and Pole...what nice about the Pole is that now I don't have to do weights for my upper body anymore) If it is a choreographed class...thats more physically tiring. It easier to demo...then coach. I'm always walking around adjusting people and such or changing equipment springs, hand placements....blah, blah, blah..
I always tell people to follow your gut, and you wont know until you try. Can you ask just to take a few more classes and be a subber? then you can try it out. I appreciate instructors just being honest with me. If she is the owner she can pick up the slack... I always do.