I am a 14 year old girl that does a lot of long distance running. is a resting heart rate of 42-48 bpm too slow?

Probably not: It sounds as if you are very athletic! Teens with a good exercise tolerance often have low resting heart rates. As long as you are not dizzy, excessively tired, or feeling a lot of heart skipping it's probably normal for you. Stay running!

Bradycardia : It is slow but seen commonly in people who are involved in sports , as long as it dies not lead to any symptoms, it does not require intervention if it is only sinus bradycardia ...