I've found this really interesting, especially the contributions by BalloonSlayer and MrsTerryPratchett thank you.
I must admit I'd always thought of flashers as sadsacks who were too passive and socially incapable to get their kicks any other way. Almost the opposite of the active aggression of a rapist. It makes sense though, the idea of starting with such a momentary, hard to trace crime and 'scaling up' when you realise how easy it is to get away with and how powerful it makes you feel.
I'm really struck by this line from BalloonSlayer: They clearly expect to you "play your part" in their vile sexual assault and woe betide you if you don't. It reminds me of something else I see discussed on MN often, to do with some men getting kicks out of forcing women to undress, or perform personal acts, in their proximity, all while being forced to 'play along' with a pretence of acceptance and suppression of signs of discomfort.
Back to running through woods though, I'd still want to investigate the provenance of the rumours. Lots of people pass on gossip thoughtlessly, others actively enjoy gossiping, many enjoy sharing or creating a frisson of fear. Many more people than one might expect believe, deep down, that women should not be alone in woods at all, that there is something transgressive, witchy, about choosing to go alone into an archetypally 'scary place', so will choose to believe anything that 'proves' this notion. Third-hand rumours do not equal truth.
My own risk assessment is that if someone wants to kill or assault me when I am out running or walking by myself, they will do, I cannot stop them. The likelihood of that happening though, is so low that compared to the benefit of the activity, I'm going to discount it. I'd take a very different view if there was a known risk in a particular time and place.