maria,
It was not uncommon for Douglas Motors to include testimonials in the back of their annual catalogs. So I had a look in the 1912 edition, and found a write-up that H.V. Swift submitted. I am not sure where the I.o.M. comes into the picture, as the route ridden was in England.
I might be that the period issues of The Motorcycle and Motorcycling carried a write-up, but do not be surprised if the same letter were used over verbatim. Anyway, this should shed a few more details on the event, in the words of the man himself. (Well, it can not be overlooked that it may have been authored within the Douglas publicity department!)
-Doug