Do as much as you can without hating your workout. Every bit of exercise goes toward your good health. It has to be a natural process that you enjoy, otherwise you may not keep up with it. Do some work with weights, half push-ups, etc., and watch what you're eating. Take each day as it comes- the more you read and practice about a healthful lifestyle, the more you're likely to change up your habits.
The folks I know who have been successful have made changes that are meaningful to them that they want to do because it makes them feel better & from then on, it's all good