I would disagree that someone with 1 year of training who runs 17 for 5k is "average talent." You take a high school team with 20-30 boys, and after 1 year of 30-45mpw, maybe 5 of them run 17 or faster. And keep in mind that HS aged kids have low body fat at that age no matter what they've been doing, even if they eat crap. And HS kids usually haven't wrecked their bodies/joints doing dumb things, ski jumping, have excess flabs of body fat nor have they been in a car accident, etc.
You take an adult on a couch to 5k program and how good can you get him? 17min after 1 year? Doubt it. Maybe sub 21 after 1 year would be great if you're lucky!!!
In the competitive running scene 17 is very mediocre, but a 17 still puts you at top 1% in a local charity 5k & may even take the victory in some local races.
Also, once you get a talented HS kid or adult with oNLY one year of experience to 17 in the 5k, if you have them do a half marathon, because they do not have much endurance base developed over years of training, they will probably struggle to break 1:20 for the half marathon, even though a 17 suggests more like a 1:18 half marathon.
The training for a half marathon and marathon at the highest levels isn't all that different. But I think concentrating on a good half marathon is a good start because the marathon requires so much, from fueling strategies to recovery after it, not to mention an inexperienced runner will usually go out too fast anyway and probably blow up on the first one and run 10-30mins slower than projected finish time.