There was a very efficient bag check in last year for any unwanted layers and keys etc. Get in the right part of the pens and you shouldn't get held up or hold others up unduly. Being slightly held up at the start can be ok, it will only be for about 800m and you will find it is a reasonable pace because of over enthusiasm by all, no matter how hard you try to start steadily!!
I tend to buy a second pair when I like a shoe so when I think it is nearing time to change, I wear one of each and feel the difference and decide if the old ones can last a bit longer. As already suggested the old ones become a muddy run or off road shoe.