I came to say what iWhatever said. There was a recent thread about children running, and someone posted that it is inadvisable for children under 10 to run long distances (over 1.5km) on a regular basis. As such, I thought that there were no longer distances organised for young children. DS1 (6) is running 1.5 km next month, and I've not seen a run for that age group and longer than that. So, I'd say YABU to let her run that far on a road at all.
I've also never seen an organised (distance) race for the under 8s that permits unaccompanied children. Not even ones that are entirely traffic-free.
Incidently, my very sporty 6 year old won't be running 1.5 km flat out, and I fully expect to be very easily able to keep up with him, usually he sprints for around a 1km then wants a sit down, but this is by the by, I don't think it's unreasonable for an adult to be unable to go out and run 2.5 hilly miles straight off.
I think you've messed up, if you can't run, or find an adult to run with her, I don't think she'd be permitted to do it. She certainly shouldn't be, if they've done their risk assessment right. unless they'll let you go with her on a bike. Which given that the roads are open, how can they stop you?