The wonderful Stadium to the Sea route that delights marathon runners, poses a serious inconvenience to anyone trying to cross between the north and south of Los Angeles, so if you're not planning on going TO the LA Marathon, you should make plans in advance to avoid having to cross the marathon route.
The most impacted areas are around the West Hollywood and Beverly Hills vicinity, where you're farther from a freeway on ramp. The folks at the LA Marathon have mapped out a few detours, but they're highly inconvenient, so you're better off making plans to avoid the area altogether.
Here is more information on LA Marathon street closures, on-ramp closures and other traffic information for Sunday, March 9, 2014.
For full marathon information visit my Simple Guide to the LA Marathon.