You start to make the biggest mistake: trying to seperate mental behaviour and physical behaviour. When there is a delayed development by no means you can seperate!
It is easy to improve physical endurance since you just apply sports physiology. Does she feel this as her problem or do you? Be aware that you do not cause more frustration about her being different. But what do you want to achieve? Is that the biggest handicap for this girl? If so then she wouldn't have theb diagnose of CP.
The symptoms, how meager described by you show to me at least 2 things: -likely problems to differentiate left from right (that is why she has problems with writing), likely poor bodyplan (delayed milestones, does not bother to comb).
How about her consentration span? I assume that is rather poor as well. You have to do a proper assessment which will take you at least 3 sessions of an hour. Starting with extended observations would help a lot in goal setting.
Read books about psychomotor development this all is controversial but you could help her a lot with getting out of the habit of approaching this as a purely physical problem. I will give you a last clue, how is her balance? If this is not so good it will influence her mental and physical performance and as a result her running will take far more energy than necessary.
Cheers! By the way I am not pretending to be right but consider what I write.