The body adapts to marathon training in a few ways. This is simplified and uses some generalizations which are not technically correct, so please don't jump on me for those things. If you want to know more, pick up a good physio book.
1) Threshold work provides a strong stimulus for the muscles to shift your aerobic/anaerobic/lactate threshold point. What this really means is that your muscles are developing the ability to process glycogen and fats with oxygen. Since there's a gradual transition from the more efficient glycogen and triglyceride pathways to the less-efficient anaerobic pathways, this training helps your marathon by keeping you more solidly in the efficient zone.
2) M-pace work is partly neuro-muscular adaptation to running that pace. Primarily, it's high-end aerobic work, with a smidge of the threshold effect described above. M-pace is a great pace to run at to provide a strong stimulus to increase capillary density, mitochondria density, muscle fiber composition (making your fast-twitch fibers act a little more slow-twitch). These things all allow you to provide more oxygen and nutrients to your muscles, more efficiently. It also makes your muscles more efficient - like a car with better MPG. This is huge, and the major way to get faster at the marathon.
3) Long runs - often combined with M-pace. These produce resistance to muscle fiber fatigue and damage, and deplete your glyogen stores to the point that the body will attempt to preserve it by burning as much fat as possible, and also storing more glycogen. It also has aerobic benefits, even at a slow pace (see above).
Anyway, here's why to train at those paces, and not faster:
1) Faster than T-pace isn't such a bad thing. It provides more fatigue and is harder to recover from, but also provides a deeper stimulus. I prefer to spread T-pace work around pace-wise, instead of hitting the same pace all the time. Most plans specify a T-pace to optimize the fatigue/training stimulus ratio, so if you trust your plan - trust the pace. It should be "comfortably hard", with deep but controlled breathing.
2) Faster than M pace isn't completely awful for someone way under-distance, who has great 5k PRs relative to their marathon. For you, however, faster than M-pace means you're providing a rather strong T-pace stimulus, adding a ton of fatigue for the amount of aerobic work you're doing. Like T-pace, hitting a small range of paces can be good. I prefer to do a smooth ramp from easy to a little faster than M-pace, so I get several miles of in-bewteen paces.
3) You can't really do long runs too fast, as long as they are between M-pace and an easy jog. Do them slowly enough to recover. If you find yourself within 30 seconds on M pace on most easy runs, add miles to the week or increase the length of your T- and M- pace workouts to up the stress level. Don't make your training paces faster - not yet.
Hanson's is really low-mileage, if it's the 30-50mpw plan I'm thinking of. That makes 3:15 seem aggressive. I'd add miles to that plan way before adding speed.