I do coach and train people. Here is the problem about doing it for fun. People value more what they pay for. It's frustrating when my commitment to others health exceeds their own. Not to mention when you do something professionally people take you more seriously. To make a long story short, my wife knows she needs to lose a lot of weight for health concerns. Even though I know exactly the steps she needs to take to achieve her goals, she would rather follow the latest fad. This time it's getting in her steps with a Fitbit. She doesn't want to hear that I know it actually does the opposite. Why? Because...I'm a plumber. People go to plumbers for plumbing and health professionals for health advice. Guess who is most impressed with the knowledge I have attained. Health professionals!
I would suggest lead by example of you want to be healthier find ways to encourage healthy diet and activities.
Some people stress eat, some have health issues, some don't excercise enough. Many different factors.
What you haven't said is: so you have any professional training or taken courses on diet, nutrition, etc? Have you thought of the profession of physical therapy?
Which is more important physical health or spiritual health?
I have told my husband I feel chubby and he says "you know how to fix that right?" I do, eat less sweets and get more exercise.
We have cut red meat from our diet, eat lots of veggies and mostly good diet, but I have a sweet tooth.
According to the body index I am normal weight and I don't really want to lose weight as much as build strength and flexibility.
We do a lot of family camping, hiking, swimming, kayaking, biking, etc.
So we are in decent shape. I only slightly concerned because I have a 5 day 30 mile backpacking trip this summer and want to train for it.
I am babbling now.
I just found I personally responded better when my husband didn't treat me like a fool and encouraged our family to do fun healthy activities. If he told me I was far and needed to workout and he would be my coach....nope didn't work so well.
We don't do runs together.he is to peppy and cheerful and encouraging. (not good when I am wheezy along...had asthma and pneumonia as a child so didn't run much....as in at all) I found it annoying and it just made me mad at him.
Anyway that is just my thoughts.
If you feel called to be a fitness trainer. then God will show you how, but your first project should be yourself. Not your wife.
As for people listening to fitness advice, my friends and family listen to my husband and i, because they know We live the advice we give to be healthy.
His parents have made an effort to eat more veggies. When I first visited it was meat and potatoes.
I got funny looks when I ask for a salad or some veggies.
Also my suggestion was not to do it for fun,but to donate some of your time to coach people who needed it but couldnf afford it. Totally different.
Charge the people who can pay, but make a point to donate your time and talent for God's glory and not to line your picketboo or buy a bigger house.