When you are only eating one meal a day, then you probably eat very close to the same amount of food each day, regardless of when you eat it; because our stomach only hold so much food.
At first, I also thought that I should have at least a protein drink before exercising each morning, but now I have found that I do just fine without it. Even on days when I feel “low-energy”, once I am actually in the water and swimming, all of my energy and alertness returns, and I really enjoy the swimming for an hour. I swim around 3/4 of a mile just about every morning, and when I come home, it is around noon, and that is when I have my meal for the day.
Since I usually have some kind of salad with an olive oil and vinegar dressing, and a protein (maybe fish); I can have all of the salad I want and not worry at all about overeating.
What I do now, is have a mug of “Bombproof coffee” (coffee with heavy cream), and that doesn’t kick me out of ketosis, but it does give me just a little something in my tummy because I take my morning meds and vitamins before I go swimming.