Dear *Name*,
Hope you're well. I am so pleased to receive a letter from you.
There are a lot of ways to keep fit. The main point is that you find an activity that you enjoy doing . There is a variety of things you can do. You can run, walk, swim, bike, go hiking, do yoga, dancing, etc. It is only for your taste.
The other thing is making sure that you follow a healthy diet. Following a healthy diet means ensuring that you eat when you are hungry and stop doing this when you are full. Also your menu has to include plenty of different fruits, berries and vegatables, seafood, beans, whole grains.
That's all for now.
Write me soon,
*Your name*