Miles 5-8
By the time you reach Framingham, the downhill course will have leveled out and it's now time to get in the zone and check off miles at your goal pace. The crowds aren't too crazy yet, so try to remain calm and remember to take in fuel and fluids, even if you're feeling great.
Miles 8-12
As you enter the town of Natick, the course is mostly flat, though there are some gentle ups and downs. Again, stay focused and try to maintain your goal pace.
Miles 12-14
As you approach the halfway point, you'll probably hear the cheering crowds of Wellesley College well before you see them. Just before you reach the college the course is fairly rural, and it's an indescribable experience to hear the distant roars of spectators as you run through the wooded stretch. I think this is the best part of the course (besides the finish line!).
Enjoy the energy, but try not to speed up too much. You still have many miles to go.
Miles 14-17
Just after mile 15, there's a steep downhill that can feel really tough. By this point, you've been running for a few hours, and that constant downhill pounding might start to take its toll. There's an energy gel station at mile 17, so take advantage of the time to fuel up and mentally prepare for the hills ahead.
Miles 17-21
As you enter Newton, you'll encounter the most difficult miles of the course.
Beginning around mile 17, you'll run a series of hills, culminating in Heartbreak Hill. Each hill on its own isn't so bad, but by this point your legs are tired and the terrain can start to wear you down. Luckily Heartbreak Hill has a great cheering section—the spectator signs are the best—so use the enthusiasm to push through.
Mile 21-25
Once you crest Heartbreak Hill, you'll encounter descents and the enthusiastic Boston College students. You only have a few miles remaining.
This area generally has good spectator support, but there are trolley tracks to your left, so at certain spots it can get a little quiet. You'll enter Brookline around mile 23, and you will officially be in the city of Boston once you hit the 40K mark. This is also where you'll pass the Citgo sign, which you've been seeing for miles.
To welcome you to Boston, there's an uphill right at mile 25 as you cross over the Mass Pike. Don't be surprised if you see many people walking here.