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.

Is swimming a hard sport?

At a casual competitive level swimming is not very hard -though I would still say that compared to other sports at that level swimming is harder. I would define a casual competitive level as ABC finals for club and JV or lower varsity (not making it

I have pain in my calf muscles of both legs when I wake up in the morning. What could be the reason and what's the cure?

"Pain" is a very general description.  If it is a cramping  kind of pain, you may not be hydrating enough. Try hydrating before bed. Drink something with electrolytes in it. Gatorade or something, especially if you're a smoker and/or beer drinker!  Caffeine hydrates you

How to calculate the amount of calories burnt in a normal day without exercise

It is a function of your Basal Metabolism Rate and your activity levels. On average your BMR is about 10 calories per body pound. If your only performing random day to day movements you would be considered sedentary so you