It gets harder after 20 miles. But all marathons get harder after 20 miles, or you're not doing it right. On the internet and in real life, opinions on the difficulty of the Twin Cities marathon course devolve into what seem like two opposing views
Since in my day job (which I should be doing right now) I'm paid to be a professional "difference splitter" I confess I don't actually see these views as total opposites. It's not a big hill, it's 150 feet (50 metres) in 3 miles (5km), and then you undulate down to the finish. But the toughest climbs on the course do come from 20-23 miles. You can't throw everything into the effort to get up the hill, because there's another 3 miles to go once you've made the climb. But you should be working harder, it should be hurting more, the further you go up the hill. The question is, how much do you give up climbing the hill relative to your pace on the flat? Conserving energy for the next 5km versus losing time?
At 20 miles you're looking for any advantage you can get, and climbing a hill is not an advantage. If you're having a good day the climb won't slow you down much, but if you're having a bad day, the hill will probably magnify your problems.
Taking advantage of living just off the course my not-secret-at-all training plan is to get in a few of my 10+ mile marathon-pace runs on the course. The goal is to simulate a bit what it will feel like to go at marathon effort up the hill. I did the first of these today, doing 7 miles before I got to "mile 20", and finishing at Summit and Snelling. After averaging 6:22/mile on the flat section I was pleasantly surprised to average 6:40/mile the next 3 miles. This conformed with my expectations that the hill is worth about a minute at my pace.
I'm hoping marathon day will be easier than today, since today I faced a 20mph headwind up Summit, and was running on the sidewalk for quite a lot of the way. After nearly 15 miles of running in racing flats I could feel the difference between concrete and asphalt...