The marathon is the "Rubik's Cube" of running. You can pull one right out of the container that's in perfect condition, and no sooner than you start to screw around with it you end up making things a mess. One that's almost impossible to return back to the original state. A runner who takes up the idea of doing a marathon without at least four years of (preferably injury-free!) training and racing of distances up to the half-marathon is asking to twist the bejeezus out of a brand-new Rubik's Cube.
The hardest part is not doing the training that's necessary to prepare for that day of 26.21876 miles, it's doing the training without becoming injured. Most of the published marathon training plans have long runs of up to 22 miles. Some longer. And most written for a general population. Look at a plan written by coaches in areas where the metric system reigns supreme and you'll find the longest run is 30 kilometers (18.641136 miles). When laid up against the 50-mile or 80-kilometer week, that single long run will take up half of the training mileage (and possibly more than half of the time) spent training. I'm more of a fan of spreading the mileage (50-to-60 miles) out throughout the week, with the longest run of the week around 16 miles, or 26 kilometers, a distance which can be covered by most trained runners in 2.5 hours or less.
Even then, there are runners who don't have large chunks of time to spare in their life. They can manage to squirrel away 8-to-10 hours during the early morning or later evening. Some ask if it's better to get the week's mileage in large chunks, say a long run of 12-to-16 miles on the weekend, and runs of 8-to-10 miles during the rest of the week. While there are some physiological benefits to pushing a 60-to-90 minute run one additional day of the week, the risk of injury increases as the runner goes out for longer duration sessions...especially as you approach that 2.5-hour mark. Add to that the amount of time it takes to recover either at home or at work and running "singles" may be a more-hazardous endeavor than first thought.
And what happens if there's a family visitor or a social function going on that cuts into that (for example) 90-minute planned run on the schedule? My take is that a shorter run of good quality is better than cutting short that planned longer effort and feeling guilty about the whole thing. I like splitting runs, both between morning and evening, and between evening and the following morning. Sure, you're decreasing the amount of time the body has for recovery, often by as much as one-half, but you're also decreasing the amount of time you're out on the run. And if something comes up on the social schedule it's not a complete disaster.
When it comes to the "long run" on the training schedule, not only do I split the run between Saturday afternoon and Sunday morning, I borrow a race execution strategy from a fellow coach, Patrick McCrann. I have the athlete run the Saturday "half" of the long run at the pace they'll run for the first six miles of the marathon. Then, on Sunday morning, I have them run the assigned distance, half at the pace we planned for the middle third of the marathon, half at the goal pace for the final ten kilometers of the race. It's more difficult for the athlete to execute the paces because they've done so much quality work from the previous Tuesday afternoon forward they're pushing five or six miles on legs that have accumulated fatigue...no different than the feeling they'll have on race day. But, they also won't be running as much during the last weeks leading into the race.
During the week, at least on the days when I haven't scheduled a particular intensity, I'm only concerned about the day's duration, which can range anywhere from 45 to 60 minutes. Anything beyond that is gravy. And it's important to recover and rest during those times in between runs; if lawn mowing or an extended house maintenance session is planned during the day it's probably better to get the run out of the way early in the day.
There may be a need for doing one or two runs of two hours during marathon training, but the fatigue accumulated through several days of quality work during the week can mitigate the need for a whole lot of them. There's a solution to the "Rubik's Cube" that is the marathon, the challenge is figuring out what jumbled it all in the first place.