Hey Tom,
I see what you are asking but you can't do it just by clicking unfortunately. That could be a good idea for a new feature if it could be done intuitively.
You can of course cut the track... or often I just use the profile tool, zoom in on the profile, then eyeball it.
Another idea is to use "Lap Analysis" to place waypoints every 0.1 miles. That's about the resolution of the cues or of a GPS/cyclo-computer, so eyeballing to the tenth of a mile is usually sufficient.
Thanks for the question.