The great part about a career in fitness is that it doesn’t require you to go back to college and making a transition into a fitness career can be done while you are currently working in your current job.
I would recommend giving yourself 2-4 months to study so look at the dates for upcoming ACE Personal Trainer exams in your area. Here is the link for exam details, and the link with dates is in the left column. There are a number of study bundles and options to give you the exact level of help you need. (I wish ACE had this many study resources available when I was getting certified back in 1997!) You can figure out a realistic study schedule based on your current schedule and lifestyle.
The fact that you have experience having hired a trainer will provide valuable insight as you start working as it will give you a first-hand perspective on what it is like on that side of the trainer-client relationship. These intangibles are things you cannot get by studying and they will greatly reduce your ramp up time when getting started. The knowledge of how to train the body is of course essential, but an equally important aspect is to know how to communicate it effectively, interact with people, and to handle all of the non-fitness parts of being a trainer.
Having managed trainers for 9 years, I can tell you that there are a lot of trainers that are good at the workout part of being a trainer, but terrible at everything else! Being a professional will move you up in the ranks of potential employers very rapidly. Another tip is to be sure to use any of current contacts and experiences in the job you have now to help you get started in a fitness career. Every work experience we have can be valuable in teaching us things to use in any future career, even if the fields are unrelated.
The hardest part will be when you actually start working in fitness. It will of course take a little time to get enough clients and generate enough income from fitness to leave your other job (if that is what you are planning on – it wasn’t clear from your question and I realize you may be only pursuing a part-time fitness career.) It is harder to go from zero clients to one client than it is to go from 10 to 20. When no one knows you, it’s harder to get started. However, after people have been working with you and having success, it gets much easier to get additional clients. And when you do get busier, if you are still working in your current job, the energy demands on you are going to become a challenge. It can be emotionally and physically draining to manage several clients all with different challenges, personalities, and programs. It is essential to take care of yourself and burnout is a fairly significant occurrence in the fitness industry.
Lastly, consider where you want to work long-term. You may start out at a large chain just for the exposure to a large pool of potential clients and to interact with a diverse array of people and personalities. However, high quality fitness professionals rarely stay at the large chains for very long since they often demand professional performance and conduct, but they rarely pay for it. And if you find you need help sorting out all of the seemingly endless options for where you can take your career, consider hiring a mentor as it will greatly streamline the process and allow you to benefit from the vast experience of established professionals. There may be a trainer in your area who can fill this role. Also, I, and many other fitness industry leaders offer mentoring services so that is also something to consider as well.
I hope this helps you get the confidence to get started as we always need more passionate, dedicated, and educated professionals in this industry.