One Possible Version:
Dear Li Ming,
I am sorry to hear that you have been ill for days and hope you are better now.You have studied so
hard that you have had no time for exercise,which is harmful to your health.I feel it would be more
beneficial to your health if you could take more exercise in future.
Taking exercise can improve your physical health and refresh your mind.After one or two hours of
exercise you can study better.Only when you have a strong body can you keep on studying without
feeling tired.Otherwise your study will be interrupted from time to time by the sickness.
Exercise is as useful and important as your study.I hope you can take at least one hour's exercise
every day after you are recovered.
With best wishes!
Yours truly,