do it if you want and don't if you don't, to be honest, if you do a bit more training and stuff (running 5km without stopping isn't loads) then you should be okay and in all honesty if your ankles are bleeding walking slower will just make them hurt longer, get some blister plasters on your ankles next time/better shoes and hopefully it wont happen again
I'm sure the teachers will learn from the map mistakes and sort things out on the proper event