The 2/3 subway to Grand Army Plaza is less than 1/4 mile from the start. The 2/3 stop at Eastern Parkway - Brooklyn Museum will be closed. The Q subway to 7th Avenue is less than 1/2 mile from the start. Please check mta.info for the most up-to-date information on public transportation. Allow yourself plenty of time, as trains do not run as frequently and have service changes on weekends.
There is also a Long Island Railroad stop at Atlantic Avenue-Barclays Center that is one mile from the race start. From points north and west, take Amtrak to Penn Station where you can change to the 2/3 subway to Brooklyn. You can also take Metro North trains into Grand Central Station, then change to a Brooklyn-bound 4/5 train to Atlantic Avenue-Barclay's Center.
If you are taking an UBER/Lyft/taxi to the race start, ask to be dropped off at Grand Army Plaza in Brooklyn,
Driving to the race start is not recommended due to scarce (and expensive) parking, road closures and traffic congestion. A parking reservation is a MUST if you intend on driving. These two garages are within walking distance of the race start and are open 24 hours.
ICON Parking: 1 Eastern Parkway. (718) 398-2518
ICON Quick Park Underhill: 105 Underhill Avenue. (718) 783-5113
Due to the race, there will be street closures from 1:00 a.m. till 11:30 a.m. on race day. There will also be parking restrictions starting the day before the race. Please see LINK for complete information