Train for Fitness (my mobile gym service) was formed in 2004. Now around 20 years ago.
Back then, I was a gym instructor and quite new to the industry of fitness.
I had entered into the industry late, in around 2000 at the age of 28 years old.
I had tried MANY jobs by then and had always kept one thing as a constant.  From the age of 18 I had always had a passion for keeping fit.
It seemed simple that turning my passion into my career would make sense to me.
"Simple but not easy."
The failure rate in the fitness industry is appalling with around an 80 % drop out rate of qualifying trainers leaving the industry within the first year.
The reasons most often sighted as unsociable hours, poor salary, burn out and even some cases, total exhaustion. 
For these reasons,  often those new to the industry will try for a term and then give up because its easier to "go back" to what they already know or what's more convenient to support their families.
They will find long unsociable hours, working in low paid gyms, as instructors novel at first but will soon come to the understanding that to become a Personal Trainer, with regular clients (year round) takes a huge amount of ground work to find upon, and a lot of on going hard work , to maintain. 

Passion doesn't pay the bills.

Even if you find a gym where you can get paid to Personal Train with enough clients to make a living, you will find in these cases their will often be a catch in that the gyms will either take 50% (+) of the money paid over the counter, per session, or you will need to pay a monthy rental fee if you would like to take a larger cut of your profit
(but you will still need to pay this fee if ill or on holiday) 
In either scenario's It requires tenacious character to make a full time job out of Personal Training.
Often backed up with extra jobs (to pay the rent) or low paid menial work , eg cleaning, within the leisure centre to make ends meet. Which will often will overall require many unsociable hours. 
A gap in the market, but is there a market in the gap?
In 2004 I formed Train for Fitness, as a small business offering mobile Personal Training for those who in particular would find it difficult to get to the gym or had NO interest in attending a gym (many people feel like this) 
I formed an advertising strategy and did a lot of leaflet drops to try and drum up extra local business. 
I offered referral incentives and slowly found my clients.  Most of which were of an older generation.  This was a good thing as I seemed to get on well with these folk.
Alongside my work In my local gym I and slowly growing business I kept up my studying throughout to allow me to work with more specialised populations, including Cardiac rehabilitation clients and more frail and elderly clients who needed to improve their strength and balance. 

Going ALL in.
In 2016 I bought a purpose selected van which would be adequate to withstand high mileage (industrial vehicle) and visit more clients (overall) , but also be big enough to carry enough equipment to cater for small group sessions and bootcamp style workouts.
Over the years since,  this proved to be a success. The vehicle more than paid for its self and with enough clients (this took time) it became a great way to offer quality workouts in gardens, fields and in people's homes.
To this day the favorite part of my business is delivering sessions from my van.. it has an organic feel about it and its been a great deal of fun over the years.
My oldest client was 99 and I believe you're never too old to exercise. 
Many of my work outs have seen me start at 5.15 AM, and in the dead of winter. So that caters for clients who are looking for an early start before going straight to work.. its a diverse service that is successful because people know you don't need to go to a gym to get a great workout.  Just great equipment, proper instruction and a good program to build upon..
Now well into my 20th year and with a virtually full diary of regular clients, this is what I wanted and have always aimed for... but I still have spaces so if you need a gym or home work out.. get in touch. 

There's no such thing as a bad work out. 

Martin