Starting at midnight, some of the San Francisco Marathon's most extreme runners will begin the 52.4-mile Ultramarathon.
Runners not challenged enough by the traditional 26.2 miles required of a marathon, will start in the darkness of night and run two marathons back to back.
The Ultramarathon runners will start and finish their first marathon at the San Francisco Ferry Building where, at 5:30 a.m. Sunday, the Ultramarathon runners will run with roughly 18,000 other runners in the official San Francisco Marathon, at which point the Ultramarathon runners will begin their second marathon of the morning.
Other runners will be participating in the full marathon, one of the two half-marathons or the 5K footrace.
The San Francisco Marathon has been newly anointed as a sanctioned USA Track & Field qualifier race for the 2016 Olympic Team Trials, meaning a future Olympian or two could be among Sunday's runners.
The marathon — in its 38th year — also features a few new additions, including the San Francisco Marathon Race Weekend App, which is available to download on iPhone and Android mobile devices. It allows spectators to track runners live and access an interactive course map, live leaderboards, messaging, social sharing and push notifications, race officials said.
A new "Finish Line Festival and Beer Garden" will be offered following the race from 9 a.m. to 12 p.m. Sunday, where runners and non-runners can enjoy beer and food truck fare.
Global live-streaming race coverage will also be taking place online for the first time this year on usl.tv.
The San Francisco Marathon is a qualifier for the Boston Marathon and now the first USA Track & Field sanctioned marathon in the Bay Area, according to race officials.
The Sunday morning race will re-route and delay many public bus lines. Specific alerts and more information can be found here. Much of the Embarcadero will be closed from midnight until 2:45 p.m. Sunday, according to race officials.
Parts of the Presidio, Fisherman's Wharf, the Golden Gate Bridge, Richmond District, Golden Gate Park, Haight Street, the Mission District and the area surrounding AT&T Park will be closed all morning while runners compete.
While there will be two-way vehicle traffic on the Golden Gate Bridge at all times, some lanes will be closed from 1 a.m. to 10 a.m. Sunday and motorists should anticipate delays, according to race officials.