Am doing my first long distance race in July, the outlaw, and was wondering having not actually done a marathon should I add this as a race at some point as part of training?
I personally wouldn't race a marathon as part of my training for Ironman.
I don't think Ive ever seen a stand alone marathon training plan that requires you to run 26.2 miles prior to the actual event, most marathon training programmes recommend you gradually increase your long run to about 20/22 miles or about 3 hours max, so I wouldn't run any further /longer than this for my Ironman training.
There's also the recovery to take into account if you did run a training marathon, you would probably need at least a couple of weeks to recover maybe even more and all that could prevent you training for your Ironman event.
Running a training marathon then an Ironman marathon would mean lots of running (obviously) which could result in injury from too much running.
Having said all that, people have successfully completed an Ironman after running a training marathon but its all down to you, if you think that you can fit it all in with your training, how quickly you can recover and how injury prone you are. Psychologically if you can run up to 20/22 miles in training you should be fine for your Ironman event.
Good Luck and happy training.