Cardiff HM was an absolute nightmare for this. Earphone wearers who were completely oblivious to other runners around them zigzagging across the route fiddling about with the device and then stopping to look for friends behind them. One girl almost tripped me up 4 times and on the forth occasion got a pretty hard shove in her back luckily I passed her and didn't see her again. When walking back to the car after we had finished one runner had earphones in and had an ambulance within a metre of him but couldn't hear it because of his music. I think inexperience race runners using their device/music are the main danger, they don't keep left or move over for faster runners. I am not a quick runner (2hr HM) and stay left as much as possible and move back in if I have past another runner just common courtesy. Earphones are by biggest gripe at all races, why can't they enjoy the atmosphere and experience of the event.
I ran my first half 4 months after having my little girl, I had not run the full distance before and the furthest I had run in training was 10 miles although I had done this a few time. I will be honest the last 3 miles were tough but I was determined to finish what I had started. I think as long as you are prepared to just run and enjoy it and not aim for a specific time you should be okay. Birmingham has a killer hill at mile 11 but the crowds are good and don't go off fast at the start as it is a bit downhill. I am supposed to be running it as well on Sunday but have had a bug for the last couple of days so I am undecided whether to or not. Good Luck
Subway Family 5k runs, brilliant races across the UK for any age, did the Bristol race this year and Cardiff last with my boys who are now 9 and 7 year old. Not sure what is left for this year but a must for next year. Great medal, tech t-shirt for every age and a subway lunch afterwards. Kids up to 12 race free and still get all the goodies listed.