It's new this year, so no, nobody will have done it. But I know the area well and it will be tough but the scenery is brilliant if you don't mind huge hills. If you want a taster of the course, the Long Distance Walkers Purbeck Marathon at the end of June - they have a running event which sets off after the walkers - follows a very similar route but you have to map read. It's only £7 (I think) compared to the Purbeck Marathon at £35.