Step up from HM to Marathon

Hmmmmm, Comparatively? That's difficult because they are VERY difficult to compare. I'm going to make a lot of generalizations in my response but keep in mind, even the same race distance is a widely different experience for people, depending on their ability and other factors.

A marathon is not simply two half marathons, much like your comment about the jump from 10K to HM (which I think have significantly more in common than the marathon/half marathon.) In addition to the challenge of running a specific pace for a specific distance, you now have the fueling variable which is significantly more important when covering longer distances.

That being said, you mentioned the longer distances being boring for you. What then, makes you want to do a marathon? It's absolutely fine to say that it's a bucket list item or simply a goal you would like to achieve at some point, but I think it's important to be able to answer that question before you start training. Unlike the other distances, I feel marathon training is really something you have to commit to. There are times when you really don't feel like getting out the door, and having a purpose is really important during these times.

Finally, for the unsolicited advice, I strongly recommend working on gradually increasing volume and consistency and incorporating it into training for some of the other distances before you move up to the marathon. Completing a marathon can be an incredibly enjoyable thing, but it can also be a really terrible experience if you aren't properly trained and I like to see people have a good experience right off the bat, and that is really easy to control with some experience and solid training. 

Asalwed said everything I was going to say plus some other smart things. She's smart like that

Asalwed said everything I was going to say plus some other smart things. She's smart like that

The jump from HM to marathon is very big. Wayyyyy bigger than the jump from 10k to HM. 

Still very doable, but you do need to be realistic with expectations. Just 'finishing' the marathon will be challenging eminently doable even without big changes to your current training regimen. 

RACING the marathon,or running the entire thing at a pace predicted by your shorter race times (like using the Mcmillan calculator, etc.) - very hard, and even harder if you're maintaining a multisport training regimen.

Another note - I know triathletes and cyclists obsess about fueling, but it's not at all about the fueling in the marathon.

If anything, I think the marathon is a great race that really shows you how not-important fueling is for 3-4 efforts, and how crucial training volume is.

I'm not saying you should plan on going in with no nutrition plan, but something as easy as 1 gel every 45-60 minutes is totally adequate for marathon racing. If you cramp at mile 20, it's not going to be because of lack of salt, drinking water instead of gatorade at the stations, etc. It will be because you outran your training. Go to a serious marathon forum and the race reports won't be anywhere near as focused on the race nutrition as triathlon writeups.

(I'm not discounting the importance of nutrition for Ironman, or even HIM races where it can def make or break you if done incorrectly. It's just that marathons are shorter and as a result your performance will typically not be made or broken by missed nutrition. Go out to ultramarathon distance and race day fueling becomes much more important again.)

Yeah, I agree. Nutrition won't likely make or break a marathon. But the longer races do put a "high demand on carbohydrate fuel supply, body temperature and the maintenance of adequate body fluids - all of which can affect race performances." So, it's just another factor that needs to be managed. Especially for someone who is going to take a longer time out there. 

yeah, I agree. That is an excellent point. Overall training load should most definitely be taken into consideration. 

Additionally, other life things like stress and sleep.