I have booked a place on this sponsored walk (I’m too old and fat to do the run) http://www.bordersar.org.uk/site/
Not a national organisation but it covers the area I play in so I keen to help them.
That looks interesting, and the 16k sounds do-able (23k might be too, but I don't know the area / route at all and it's fair to say I haven't walked that far all at once since before my 5-year-old son was born).
If you'd rather organise something ourselves, there's a couple of options in the South I can suggest. Some of these are quite long, but we can always shorten them depending on popular demand. They're all fairly low-level so they should be practicable in the autumn (and the winter, if you fancy that sort of thing).
How about a sponsored Ridgeway
walk from Overton Hill to Streatley (or vice-versa, depending on whether faerie9 wants to start from home or finish there
) It'd involve an overnight or two since it's 43 miles, but I've walked it before and it's fairly easy-going. Streatley's got a railway station which makes getting to/from that end fairly easy if you're relying on public transport.
Or there's the Wayfarer's Walk
which also starts near Marlborough but then runs to the South coast. A 15-mile section should be easily done ina day, or we can stretch it out if you fancy a night (or more) on the trail.
And then there's the Hangers Way
, which I don't recall having walked upon but is 20 miles, starts at a railway station and very nearly finishes at one too (there's one at the 15-mile mark).