The body can only convert so much fat into energy at a given moment. Usually we have both carbs we've recently eaten to fuel us plus a store of glycogen. On Keto you should have little to no glycogen and only whatever small number of carbs you've eaten recently. If your energy expenditure exceeds what they body can convert from fat along with whatever it can extract from your limited carbs and whatever glycogen is hanging around, you'll get an energy crash. Keto might be good for ultra endurance events where there is little to no sprinting involved. Just keep a nice steady pace at the level that can be fueled by the body's capacity to convert fat into energy.