One possible version:
Jane,
I' m very glad to receive your letter.But at the same time,I' m sorry to hear that you are not very well
these days.In your letter you wants to know how to stay healthy.Here is my advice.
Firstly,you should keep a balanced diet.You should avoid eating food high in fat and too much sweet
food.You' d better eat more fresh vegetables and fruits.Secondly,taking exercise every day helps building(up) a strong body.Why not take more exercise in your spare time?As we all know,regular exercise is
an important part of keeping us healthy.Finally,make sure you have enough sleep and you mustn' t stay
up too late at night.
By eating properly and exercising regularly,you can keep your body at a proper weight and stay
healthy.I truly hope my advice is helpful and you' ll get well soon.
Best wishes!
Yours sincerely.
Li Hua