What are fitness mistakes you always see people make?
The biggest mistake is the more is better philosophy. Train to get stronger and run to get faster. Don't just set the timer and train X hours. Pointless. If you want to improve the way the body looks then improve the way the body performs. People don't get how running one 800 meter sprint after warm-up twice weekly is superior in the long run for lifetime leanness than a gazillion mind numbing miles. People don't get that you could literally just only do 3 weight training exercises ( 1 push, 1 pull, and 1 full range leg exercise) each workout twice a week and you will make fantastic gains as long as you try to get stronger each workout.