Lee - 3 weeks in may or may not make you fat adapted, particularly since you are hitting the wall after 45-60 minutes. I hate to say this, but you might consider your usual carbs for this very hard competition and then back on keto the next day. Would love to know how you do later on when you are definitely fat adapted. You can be doing keto for a while before you really are fat adapted.
Lee, this is just my 2 cents worth and it may not even be worth that much.
Anybody else want to chime in?