FINALLY, one of the major aspects of muscular strength you will need to acquire is that of being able to "yomp" (or maybe this is just RM, as we do have a tradition in it and a dreadful 30 miler at the end of training), something that you cannot train for outside of the Armed Forces (short of putting on a bergen full of rocks and walking for 8 hours).
I believe the paras call it TABing (As in Tactical Advance to Battle)
If you decide to go for a lower key race I cannot recommend Abingdon highly enough, flat course, well organised, park 5 minutes from the start/finish. see your supporters 3/4 times did I say well organised?
I might be a little biased as I have set 2 pbs there out of 2 starts though!