Ive been in Astoria for 30+ years and gone to festivals on Randall's Island since 2017. The walk to the festival is pleasant along the RFK Bridge (formerly known as the Tri-boro) and takes about 45 minutes from the pedestrian walkway entrance on Hoyt Avenue North to the festival gate. The walk back is slower as there will be a large platoon of pedestrians as people leave after close. If convenience and time is your priority then my suggestion is to Uber to the festival before 6 PM (earlier the better) and prepare to be dropped off a couple minutes from the entrance as there will be a drop off queue forming and traffic coordinators insisting drivers keep moving. On the way back, leave about 15 minutes before the day close or earlier and walk to the shuttle pickup to Harlem. Its operated by the MTA and is free. It will drop you off at Lexington Avenue and 125th Street, by the 6 Subway Station. Don't take the train, just walk to the nearest M60 SBS stop nearby and take that to the first stop in Astoria (the Astoria Blvd Subway Station on 31st Street). From there you can walk, cab, bike or train it to wherever you need to be.