Last week, Google sent tons of warnings to personal trainer websites that have "critical mobile usability errors" (see the message Google sent me for one of my sites above).
According to forums geeks like me frequently visit, these were just warnings and they don't mean you site will be penalised. However, it may be time to refresh your knowledge of what makes for a mobile-friendly site and get a mobile plan before Google gets more serious about it (and before it adds more weight to the mobile-friendliness ranking factor).
Check your website - First things first. Is your personal trainer website already mobile optimised? You can check how mobile friendly your website is according to Google here:
Google Mobile Friendly Test
You'll either get some good news, that your site is mobile friendly or you'll be told that your site isn't and why.
Pick a solution - If your site isn't mobile friendly then you'll need to choose one of the many solutions that are available.
Google recommends that you build a responsive website but there are a lot of other acceptable (and cheaper) options that will work just fine: