Diet is 90% of general fitness and health. You don't need to step foot in a gym if you eat right.
A lot of people know how to eat right but they choose not too. It's not always the easy choice this why people generally eat bad.
A lot of people can kill it in the gym. People think their workouts aren't working out because they don't see results. If you compile Killin it in the gym with eating healthy your looks will skyrocket.
Between maintaining healthy eating and killing it in the gym people tend to taper off. Might be good for 3 months then

.
Most programs will give you the info of how to get there but then once your there is like

what now? How do I maintain?
Being fit, in shape and eating right, exercising isn't a 3 month, 6 month, 6 week deal. It's a lifelong commitment. When you look at it like that it's very overwhelming. Most people struggle finding the time of day to eat right or workout let alone do both, and if you try to plan it out for the rest of your life good luck, it's tough. So they just give up.
You gotta take it one day at a time and focus on 100% completion today. Then when tomorrow comes focus on doing it again.
Itll become natural and routine. You'll learn how and when to give yourself breaks, rest, cheat meals etc.
Another hard part for people is just getting started. They do wayyyy too much prep work. Just start. Walk in to the gym, today. Put your phone down or laptop down and do 50 Push-ups NOW. Do 75 bodysquats now! No "imma start tomorrow" no "imma wait til Monday so I can do it Monday, Wednesday, Friday. NOW! No " I gotta buy protein powder or vitamins" NOW! No "I gotta buy weights, or a membership" NOW!
Turn "No" into "NOW!"