I get exactly this too! My physio suggested a few stretches to do before/after that have made a real difference for me. Kind of hard to explain but I'm sure you would find something if you googled neck/back stretches.
Not sure the long run would need to be of half distance if that's what you mean. I'm 17 and do about 45 miles but my long run is no more than 70 mins which works out at about 11-12 miles.
Also, I agree that most 16 years old could probably happily do a half marathon with a little training!
According to the BMC ( who admittably are looking more at middle distance) 10-15 weekly miles would be ideal for a 13 year old. The recommendations from 13 onwards are:
denotes Subscriber-only content | 
JonnyD
