Hmmmm.... what are you eating on the first day? I find it pretty easy to go back into keto in less than 24 hours, even without doing any cardio first. Just cut out your carbs and eat tons of fat. While different people have certainly levels of resistance to going into keto, this seems to work for me very consistantly. Once you have gotten used to doing keto diets your body should adapt, and accept it as a natural state. Then again, I do low carbs for bulking too, so my body is just just to useing fat for fuel.