The first couple of miles are downhill. You're also running with a very large densely-packed group who are just as fast as you. It's very easy to get sucked along to run much faster than you planned for the first few miles.

The Newton Hills, esp. Heartbreak Hill, are near where many people will hit the proverbial wall, distance-wise. Having to go up ever-steeper hills at the same time can be really taxing.

There are so many things that can go slightly wrong, and when you have to endure those slight inconveniences for 26.2 miles/2+ hours, it can feel like an aerobic Chinese water torture.

The year I ran, we had a tailwind, so I had an relatively easy time of it.

Great friendly crowds.