Long Road Ahead

Long Road Ahead is MacCready's companion quest in Fallout 4.

Detailed walkthrough
When traveling with MacCready long enough for his affinity rating to reach between 250 and 500, MacCready will talk to the Sole Survivor and tell them about Winlock and Barnes, and how he wants the Sole Survivor to go to the Mass Pike Interchange and kill them. This will start Long Road ahead, and Mass Pike Interchange will be marked on the map (MacCready's affection gains will cap at 499 until the Gunners portion of this quest is completed).

When Mass Pike Interchange is reached, both Winlock and Barnes will be on the interchange itself, so locating one of the nearby lifts is necessary; traveling through fast travel will bring you to a lift north of the interchange, saving the time to find one. Resistance in the interchange is fairly heavy, with 10-15 normal Gunners, a commander, and one assaultron. Either of them may also be using a set of T-60 Power Armor at higher levels. After both Winlock and Barnes are killed, MacCready will talk to the Sole Survivor and say that it relieves him to see the two responsible for harassing him for leaving the Gunners be killed, and return the caps the Sole Survivor used to hire him. Actually taking MacCready himself to Mass Pike Interchange is not required.

After MacCready's affinity reaches 750, the second part of the quest will begin. MacCready will talk to the Sole Survivor again and tell them about his son Duncan, and how he has a disease that will ultimately kill him. He then asks the Sole Survivor to visit Med-Tek Research, where a cure for the ailment may be found, and that he has the containment override password. When there, the cure is believed to be housed in the lower levels, which requires reaching the top floors of the facility to the executive terminals to override the containment. Overall resistance is fairly low, as the feral ghoul occupants (including a glowing one) are spread out across a wide area.

After finding the cure, located in the lowest sections of the facility, MacCready then tells the Sole Survivor to bring the cure to Daisy in Goodneighbor, where she will send it on a caravan out of the Commonwealth. After giving Daisy the cure, the quest will be complete. MacCready's affinity gains will cap at 999 until the Med-Tek Research portion of this quest is completed.

Bugs

 * Winlock may be encountered in an inaccessible area, on top of the upper freeway platform, rendering the mission unable to complete.
 * Possible fix: Move south, one then can access the upper deck by a collapsed structure forming a ramp, and jumping a small gap from girder to girder to finally reach Winlock.
 * Having already cleared Mass Pike Interchange the quest may not start when bringing MacCready there.
 * Winlock and Barnes may not be hostile to the player character at Mass Pike Interchange, even after they kill all of the other Gunners at that location. Even if the player kills either Winlock or Barnes, the other may not be hostile.
 * MacCready will add the location of Med-Tek Research to the player's Pip-Boy. However when selecting 'Show On Map' for the quest objective 'Bring MacCready to Med-Tek Research' the map will not center on the location. There will also be no objective marker to direct the player to the location (this bug will not affect the quest's progression, once the Sole Survivor reaches Med-Tek Research it will progress to the next step of the quest).
 * "Locate the Executive Terminal" will sometimes not update when the player reaches the terminal. Which also means MacCready will not give the player character the password.
 * Reloading to a previous save may fix this.
 * Another possible fix is to use the console command.
 * If one picked up the Prevent even before making MacCready a companion, the easiest way to fix it is to do ; the Sole Survivor and MacCready will be partnered up, show him the Prevent, and take it back to Daisy.
 * A glowing one inside the room with the Prevent can spawn or walk too close to the terminal and door, prohibiting interaction with the terminal to open the door even though there is an intervening wall - the game will go through the animation of aligning with the terminal and begin to zoom in, then suddenly abort and return one's control. There are two possible solutions to this issue.
 * Reload from a save prior to entering the sub-level elevator and hope the offending ghoul doesn't trip the bug again.
 * Open the command console, focus on the door, then it. After clearing out the ghouls,  the door and use the terminal to open it as intended to also open the shortcut exit. If the door is still disabled, the open prompt will not appear on the terminal and the shortcut cannot be opened.