panad - so, which Brutal is it to be? Half, full, double, or triple? Or for a cool £9995 the deca?? (I think you should be given food and accommodation at that price, e.g. a bus.)
Thought that must be, brer. You were very quiet about it in your first post. You will have to get your nutrition/fluid intake sorted. Something awry there. You don't seem the nervous sort either, so not exactly start line nerves!!
With EnduranceLife I'd go for the Flete marathon - it's later in the year and only category 3 (strenuous) in their category system. The South Devon one in early February might not be a good idea for a first one and is graded 4 (severe), whilst the North Devon one (Exmoor) is '5' (extreme).
In Flete there is quite a bit of pleasant running through woods and a very interesting 600m estuary crossing. Be sure to consult the tide table to find out how deep it will be! I think though that the organisers plan the date of the event based on there being low tide at this point.
The good thing about their events is how well waymarked they are. They even have signs with crosses on on the paths you are not supposed to use. Hard to go wrong.
You'd be better off going for speed, I think, and developing a good running technique. You would probably take about 2 hours to do a 10K and if you entered a race you might find the whole experience not very good for morale.
Even 5K might be too far. Warm up, run for a mile at as good a tempo as you can manage and then cool down. Concentrate on good leg turnover.
I'd read as much as you can before the event. That Ch 16 gives a brief outline of the logistics when Sir RF and MS did theirs. It was a privilege to run with them on their London leg.
It doesn't look like you will have International Date Line issues which helps. They effectively did seven marathons in six days. Also you do not have the deadline of official marathon races to fit in with, which they did in Singapore and New York.