You'd want a decent day's flying...by that I mean at least 3 hours in the air (plus then people renting could justify taking an aeroplane away). Except 3 hrs for me is 300 miles, for someone else it might be 500 miles. If you designed the leg for them, it'd be 5 hrs for me! You could always break it up by having intermediate stops but you'd have to visit a lot of airfields in 12 days to actually make it take 12 days.
The Around Europe would be a brilliant idea - often a lot of people are scared to venture to Europe, but if it was part of an organised trip with PPR's etc., pre-arranged I reckon a lot of people would take part. The Euro one would certainly be more challenging from a aviation point of view.