Greg responds...

I can't answer this question easily. For starters, Wyvern is a fictional location on the West Coast of Scotland. So where exactly are we measuring from? Secondly, Loch Ness is about thirty miles long. So where exactly are we measuring to? And are we measuring as the crow flies or along practical routes that a car or horseman might have to take?

Nevertheless, I'll give it a shot. As long as you understand I'm not being held to this. In particular, I've done no research to back up this stab at Wyvern's location.

I'm taking out my map and I'm measuring (roughly) the distance as the Gargoyle glides between the Point of Ardnamurchan (at the western end of a western Scotish Peninsula) and Castle Urquhart (which I believe is the model for the fictional castle where Sevarius set up his operation) on Loch Ness.

I come up with approximately 83 miles.