I think the Reuters story below has a major error in it. Considering Tokyo is like 130 feet above sea level, I don't think this could possibly be true.
The final stretch promises to be gruelling, with the athletes finishing on an incline averaging over 10 percent over three kilometres leading back to where they started at the Olympic stadium in the city's west.
That would be the most incredible finish in marathon history. A 10% incline over 3k? That would mean the course would rise 300 meters - nearly 1000 feet - over 3k. What are they going to do - have it finish at the top of a sky scraper at the Olympic stadium?
Ok, I've found the actual elevation map for the course. I was correct. Reuters is wrong. See for yourself.
The course will rise about 30 meters from 37 k to 41 k so that's about the same size as Heartbreak Hill (27m) but Heartbreak Hill takes place over the span of just 2km roughly so this is about half as steep.
Here's a video of the course:
And an Athletics Weekly article:
And the Reuters one which seems to be inaccurate.