Some great advice in this thread already. What I would suggest: make your clients your sales team!
I’d suggest you should focus primarily on your core business. In personal training this is doing a great job (i.e. getting great results) with the people you are currently training.
As far as marketing goes, referrals from your current clients will be the cheapest channel possible, and so by doing a great job with those training with you means you do not have to immediately spend time and resources on other marketing activities.
Think of it this way: if you do a great job with your clients, then they will be selling your product for you every day! They may do this directly (e.g. telling friends) or indirectly (e.g. people noticing them performing well or being in great shape).