True story: in my first marathon, I ran 26.7 miles by mistake. WHY? I didn't get lost... It's because I didn't know about TANGENTS!!
A tangent is the shortest distance between 2 points, and if you run good tangents in the race, you'll end up running the distance of the race but NO MORE!
Certified race courses are measured so that the MINIMUM distance you will cover is the official race distance, but this means if you weave around a lot, or hug the curves of the road going back and forth, you'll end up covering more (sometimes a lot more...) than the actual race distance.
Let me tell you, there is NOTHING so demoralizing as realizing that you could have hit your race goal if you had run SMARTER tangents!
Now, in some races where the roads are left open to traffic, it's harder to run perfect tangents because you have to follow the road more closely than if you could use the whole road. But if you're in a race where the roads are closed, you can definitely run smart tangents!
A big barrier to running good tangents is that they aren't exactly polite. If you run good tangents, you'll end up running on both sides of the road. You might end up cutting in front of people. It can just feel weird to run good tangents. So that's why you should practice, when it's safe to do so!
HOW TO RUN GOOD TANGENTS
Don't weave in and around other runners (this can add SO much distance and you won't even notice until it's too late)
Take notice of the course ahead and chart the straightest line forward possible through bends and curves
If the course has turns, pay attention: if your next turn is to the same side (right turn -> right turn), then stay close to the curb on that side. If your next turn is on the opposite side (right turn -> left turn), run a straight line diagonally to the next corner.
Know where the water stops/aid stations are and what side of the course they are on so you can plan ahead
Don't panic! No one runs *perfect* tangents; the goal is to get as close as possible!
Just like everything else, running good tangents is something you will get better at with practice and experience!