Lethargy stems from the mind. Studies prove that rarely, very rarely does physical nature of work tire us. Even the tiredness we may develop, is usually gone with proper rest.
What tires us, is the mental resistance to doing something. Your brain will find every possible mediocre excuse to not do it. It is the nature of the mind. It resists any change. The Law of Inertia.
However what you can do is, use the same Law of Inertia to your benefit. If starting something is tough, then according to the same law, stopping once you have started is also tough. The cue here is to start. Start doing. One small step, then another, then some more. Don't get bogged down by unnecessary details and the number of steps you have to take. Don't think. Focus on that one small step. Give it your entire attention. Slowly you will gain momentum. Then it becomes easier and easier.