Erin Hills is about 7800 yards of golf course from the tip, but as Jack mentioned, it's a 15,840 yard walk. A very nice older course I'm familiar with is 6,400 yards long, but around 7,400 yards to walk.
What is the maximum excess yards of walking to give a course a bad routing rap?
I think the additional yardage at Erin Hills has been a bit overstated in this thread. Here are the individual hole transitions:
Start with the walk to the first tee, which is not that bad (its right off the practice putting green). The walk from 1T to the 1st fairway is a little bit of a roundabout, but its not that far off a direct line.
1G-2T - a 40 or 50 yard walk back to the 2nd tee, but all in all not egregious, especially since you can leave your bag.
2G-3T - a short uphill climb to the 3rd tee, but nothing unduly taxing (I believe there are new back tees to the left of 2nd green which make the commute even shorter and take out the hill climb).
3G-4T - another short climb, and certainly nothing to complain about.
4G-5T - the worst backtrack on the course, brought about by the removal of the old green, this one I'll concede.
5G-6T - a lovely transition
6G-7T - with the removal of the dell hole, the new par 5 7th tees are a short walk from the 6th green (less than 40 yards).
7G-8T - you have to navigate the most confusing crossroad on the course, and it is perhaps a 100-150 yard walk.
8G-9T - 9th tees are just off the 8th green
9G-10T - a 100 yard walk from green to tee, but its level.
10G-11T - a bit of a trek created by the removal of the old green, chalk this one up to tinkering, not the original plan.
11G-12T - a quick jaunt to the right of the green, certainly nothing out of the ordinary.
12G-13T - another easy transition.
13G-14T - this one gets a little wierd depending on what tees you are playing, but nothing outrageous.
14G-15T - a particular favorite of mine, with the tees perched above the 14th green offering a view of the hole you just played and the hole to be battled next.
15G-16T - a very manageable walk.
16G-17T - with the back tees just next to the 16th green, this an easy one.
17G-18T - actually not as bad as most think.
In general, the walks from tee to fairway are direct with very few obstacles that need to be navigated. The major issues (4 to 5 and 10 to 11) are the result of the recent changes to the course. If blame is to be laid, its that those responsible for the renovations did not do enough to take into account the effect their changes would have on the walking golfer. Another situation where daily play lost out to the once in a lifetime experience of hosting a major.
For the first timer, the navigation process may be a bit daunting (knowing where the 2nd tee is, how to get from 7 to 8 and perhaps one or two other minor mysteries). But if you've played it once, unless you lack an internal compass and any bit of memory, there should be no reason not to be able to get around the property in a highly efficient manner, unless, that is, you're spraying the ball from one side of the fairway to the other.
I've played the course carrying my own bag in less than 3 hours with the occasional wait for other players. Its not that bad.