When Jesus was fasting in the desert, there is no doubt that it made him physically weak. Yet, the fasting and closeness to his Father God obviously emboldened him spiritually. Despite the hunger and the weakness, he was sharp spiritually as he quoted Scripture, denied Satan a victory, and held faithful to his mission here on Earth.
Fasting actually made Jesus tougher, stronger, and more able spiritually to get the job done.
I'm praying that as your physical energy may go down, that you would experience revitalization spiritually as a result of the fast. We're in the middle of a fight, and we need all the spiritual stamina and strength that we can get!