Fix grammar: Dear Bob: I think, If you enjoy your new job you must stay here, but you have to do exercise within your free time, and you must do too a good diet, because upon you doing not did any exercises and not do a diet you will having problems in your health, try to do it, and including you can't do it you must leaving your new job, because if you keep all the day sittin on a chair you will have a lot of problem with your health, please, trying to do more exercise and a good diet.
Dear Bob, I think if you enjoy your new job you must stay there, but you have to do exercise in your free time, and you must have a good diet too, because if you do not do any exercise and do not have a good diet, you will have problems with your health, try to do it, and if you can't do it you must leave your new job, because if you keep sitting on a chair all day you will have a lot of problems with your health, please try to do more exercise and have a good diet.