How long would you class as limited? Is this not the same as saying get your enties in now or you wont get in. Would you send out an invitation by e-mail along the lines of " An entry has been reserved for you, if you don't reply within 4 weeks, the organisers will assume you do not wish to run?"
Incidentally Marvin, as well as trying to beat previous times, don't we also have a scan of 'faces' running alongside and just in front of us to look out for regulars that have similar times or better so that we mentally see them as targets to beat?