Janrg1, the being energised comes when your body is used to the lack of calories combined with the good effects of highly nutritious food. On day 2 of the diet you would be feeling shattered, your body is missing the bad carbs because you were addicted to sweet. It is like giving up coffee or alcohol, your body has a little tantrum and stamps its feet. I would suggest you continue to exercise but be kinder to yourself and take it easy. Around week 2 you will be skipping and jumping. Let us know how you are doing.
You also say it was hot, anyone would find exercise a challenge in heat they are not used to. Fear not it would come. You don’t say if you are diabetic and if you are you will know you have to take care not to hypo and count a few snacks into your daily 800 calories, carry a few nuts or raisins if you are out walking, drink lots and lots of water and keep well.