If you do things right, and you have the business savvy to back you, you can make trades with they gym owner. Help him manage for free, collect 100% or find a gym where you are the only trainer and the median income for the area is over 6 figures.
We aren't all stupid and broke.
You learn as you go. Make contracts, learn to sell, be good at what you do so you get the referrals on top of it.
I have a waiting list and have had one for over 6 years.
Good luck if you just got an online cert and hired on with a corporate gym. You may as well work at Mc Donalds. You are right, those gym owners are laughing especially when they tell you that the major objective of your job is to do 25K per month in sales... especially in a city that is on the verge of bankruptcy.
Work where people have money and know WTF you are doing. If you don't but you think you have the ability, pick up a side job until you get there.