Running

18mile is a long run, especially at just below 3hrs. very few elite marathon runners would train over full distance. i severely doubt you will get any benefit other than increasing chance of injury.
this is from a junk mileage king, in the past ;)

I know what you mean, i'm basing my training on a lot of what http://www.mcmillanrunning.com/ who advises long runs should be at maximum 45 minutes over target time.

i just want to see how it feels, i'm aware that i need to concentrate on avoiding junk mileage as much as possible, so have been doing my commuting jobs as a kind of fartlek training, and doing a few speed runs, but basically i'm making it up as I go along., so if you have any free/cheap sources of advise on how to train then please point em in the right direction.

The other reason for the run is that i really really really enjoyed my run on sunday, it wasnt easy (i set off at 1pm) so was very hot, but i was in the zone almost the whole way.

Basically it's better than any drugs i've done.