for anybody interested i found a partial answer to this post, i measured my first checkpoint on the virtual 100 as exactly 10 mile, using the measuring tool, my son double checked this distance on his strava which measured it at 10.4 mile
based on this i would have done well over 100 mile unfortunately covid didnt get me but my second vaccine did as i felt pretty awful from the off and had to pack in at 33 mile ,feel a little cheated but have the bullock smithy as hopefully a qualifier.

Could anybody confirm that the measuring tool on bing maps (and other map apps) is accurate enough to establish distances for the virtual 100 ,i have found with the zoom applied you can get an accurate
tracing of the road/track/ footpath to be walked.
regards