I found that going lactose free helped me avoiding trouble on my runs. For events I still take immodium but since going lactose free it was also better for my last HM. Good luck in finding out what works for you. It's not nice having these problems when you are running.
You got some very good advise there. You are very keen which is good but please listen to your body. 20 miles in 8 weeks is a hugh ask for your body which will put you at risk for injury, let alone trying to do it at the pace you are aiming for. Your body needs time to adapt especially as you just started running.
Thinking back, my primary school (for 6-10 year olds) didn't have a fence. Once classes were over we would just make our own way home and the parents didn't have to pick us up. We would also make our own way to school and not getting dropped off by our parents. The other kids would take the piss if they saw you being dropped off by your parents. I normally cycled the 3k to school as there was no bus from our house to the school. Haven't been back for a closer look for years so can't confirm if they put a fence up in the meantime.