Travel Insurance for Specific Sports Trip

HI all - I am going on a holiday later this year to participate in a sporting event (marathon) and nothing else - essentially fly in, run and fly home (plus a night in a hotel either side of the run). As the trip a bit offbeat the costs are relatively high, so ideally I want an insurance policy that would enable me to cancel the whole trip if I was ill or injured which meant that I could not participate in the event.

There are plenty of insurance companies that will cover participation in sports or extreme sports, however if for example I was mildly ill with a bad cold or sprained an ankle in advance of the trip, it is likely that a doctor would advise against running a marathon, but probably wouldn't advise against travel in general (e.g. getting on a flight) - most travel policy wording requires a doctors certificate stating that you are unfit to travel before paying out under the cancellation clauses, so in these circumstances I would end up getting a flight, sitting in a hotel for a day or two and flying home.

Do any forumites know of any policies which would cover cancellation where the policyholder would be able to claim for cancellation if unable to participate in the sporting event that is the primary purpose of the trip even if they are well enough to travel?

Sorry for the long-winded explanation and thanking you for any help in advance!

Mark