I'd actually welcome that at the best of times. Would be like being 'promoted' or 'relegated' each time you moved group!The way they've been doing it is to divide the entrants into groups of 6 by expected finish time & starting each group 5 minutes apart to minimise overtaking, fastest groups first so a smallish race of say 250 people will be broken down into 42 micro-races. They could probably get the start groups down to about 2 minutes apart for shorter stuff like 5K.