This is a complicated issue. First how many grams of carbs are we talking about? Percentages can be highly misleading.
Also, how much protein and fat are we talking? How much protein are you taking in now? (My guess is you have no idea). There is little evidence that more than 2 g of protein per kg of body mass is needed although if you are going low carb that might change things. Going too high can present other problems if you have an underlying kidney issue.
Also your body might have trouble taking in more than about 30 g of protein in one sitting or meal.
The first two weeks on this potentially low carb diet you are likely to feel like crap. The body adjusts after that in most cases, but typically folks up the fat.
Your body can adjust to burning fat during exercise and it can make glucose from some proteins like alanine, but it is a messier proposition.
Once you are adjusted to the low carb you may be in a ketogenic state. This might not be so bad if you are really overweight. I will predict that even after adjusting you are going to find your faster pace runs much harder to do. Your long runs likely will also slow although they might not slow enough to matter.
To run fast, I think you need carbohydrate (stored as glycogen). Whether 25% (again the percentages can be very misleading) will give you sufficient carbs depends on your training and just how much in actual grams per kg body mass that it.