First of all, we recomend you take public transit no matter what. mta.info is your friend. Google Maps is also your friend.
To get to the first packet pickup event - the NYCRUNS Holiday party, take the L to Bedford Avenue. You can also take the G to Nassau Avenue
To get to the 2nd packet pickup event at Brooklyn Hospital Center, take the BDQ to Dekalb Avenue
To get to the 3rd packet pickup at JackRabbit Sports in Park Slope - take the F/G to 7th Avenue, the Q to 7th Avenue or the 2/3 to Grand Army Plaza
To get to the start take the F/G to 15th St Prospect Park or the Q to Prospect Park. The 2/3 to Grand Army Plaza is a little further.
Please check mta.info for service changes - particularly on Saturday and Sunday.
If you are driving - please figure it out yourself. There is pretty much only limited street parking at any of these locations.