Thanks for the reply, appreciated. I'll make sure not to leave anything valuable.Looks like it's a 3 lap route with start and finish close together.
At any parkrun ypu will find a lot of people leaving things in the start / finish area, even tying things trees and fences. It will be a busy area and things left are generally OK.
The volunteers will be busy with their designated tasks so won't have time to act as security for anything left in the area.
I have previously fastened a similar bag to my bike and it was OK. Just don't leave valuables.