Thanks for the informative post, Zouse. This is also covered in the book with a somewhat misleading title "The Sports Gene" by David Epstein. IIRC there is some discussion of the separate aspects of high trainability vs high VO2 max. When both of these occur in the same person there is the potential to reach elite levels.
If you are looking at Salomons your could try one of their superseded road-to-trail shoes. You should get a good price on Sense Mantra 2 or Sense Mantra 3. I've clocked up nearly 1500 km on 2s on a mixture of road and trail. So long as there's no mud to clog up the shallow tread, such hybrids could work well.
There are a limited number of entries but you have to go with a tour company that has places allocated, I believe, Cougie. Next year's prices are not announced yet, but a trip to the Royal Parks half, which might be comparable, would cost me about £890 all in. I guess I'm spoilt by Boston, where I simply need to be quick enough and I get in and arrange a (better value) trip by myself.