I’m going to offer a different opinion, based on my own personal experience — I started this way of eating eight years ago and have continued with success since then. I think it is really important to discover (through trial and error) what works for YOU and your specific body/metabolism. What works for one will likely be different for another! All that we can offer is some advice we learned from our experience, but you must figure out your own path for yourself.
With regard to exercise, I have two important points (I am “sporty” like you and I truly enjoy several hard workouts each week. Probably like you I burn off 500-700 calories per workout).
The first is, when you start this way of eating, you might find that you need to cut BACK on hard workouts for a couple of weeks as you adjust to it all. I could not even do the harder workouts for the first few months, and I found a lower level workout was still quite effective. I was surprised by this, as I have used vigorous exercise my whole life to control my weight (not very successfully). What I discovered was, that it is more important to eat properly than to exercise vigorously. So cut back if you are feeling hungry/ overwhelmed/exhausted and realise you will be able to go back to harder exercise soon (your body will tell you when it is ready).
Secondly, yes, absolutely, for ME, I HAD to add in extra calories because I simply could not function on the 800 calories and also do a workout. That has always been true for me. I don’t need to supplement all the calories I burn off, but at least a couple hundred calories in the form of extra protein (chicken, tuna, egg, nuts — only ketogenic friendly foods). This kept me sane, calm and reasonably fed while not sacrificing any progress.
Your goal is to keep your insulin levels down while still fueling your muscles — as a sports person I would strongly recommend Dr Benjamin Bikman’s podcasts about insulin and exercise. They helped me so much! Just Google Dr. Bikman.
So keep going! This way of life is incredible, sustainable, fun, tasty, healthy, life affirming and your brain will thank you when you are 80 years old.