If you look at this project (http://projecthorus.org/?page_id=1773
), the distance traveled was 111.5 km. The balloon goes up rather quickly, so if the wind is as quick as that you travel 30km upwards under 45 degrees, then down quicker with about 5 m/s.
And if you launch a sail plane (glider) from the balloon @ 30 km height it could travel something between 500 and 1000 km before touching down. According to my not-so-scientific calculations, that is
Ah! but IF the glider was in fact an autonomous drone, controlled by the Pi. following a predetermined flight path back to its point or origin (or close to it), then the recovery crew wouldn'y have nearly so far to walk, and the 'Pi would have something useful to do during the descent...
I'm not sure what the CAA etc. would have to say about it though - we may all have to decamp to Spain after all