There is no specific cutoff time for the marathon itself. The marathon cutoff, is the race cutoff -- 17 hours -- midnight. Get over the line by 11:59:59, and You. Are. An. IRONMAN!
You'll likely do 2-3 stand-alone runs and 1 brick run (right off a bike) per week. These will be some of your most taxing sessions - but strength here, gets you over the finish line.
Most beginners will 'hit the wall' during the run. The part where even walking is hard, where you question your sanity. Mentally, this is the hardest part of the race. The saying goes "Ironman starts on the back half of the marathon".
You'll need (several) solid pairs of running shoes for training, and a fresh pair for racing. Add a bib belt, and the rest becomes optional.
Aid stations at Ironman races are like buffets. Eat what you need, just before you need it. And if you're all out of steam, maybe try a sip of Coke or Pepsi. Sounds crazy, but to some, it's a life saver.