Well I found myself spending more time wandering the northern side of the map rather than pushing the main quest onwards.
I reached the point where I should be handing a cigar to dogmeat to smell, but I got side-tracked trying to work through the growing list of quests I'd already picked up....
Now looking at my stats I see I've discovered 168 locations.. And that after 62 days of wandering the wasteland my completed Quests is 46 and breaks down into 25 miscellaneous objectives, 3 completed main quests, 18 completed side quests, 10 BoS quests and 13 Minuteman quests....
Companion-wise, I've found: MacCready (a former lost lamplighter kid from FO3 turned sniper/hired gun), Curie (the French medical droid), Caite (the fighting irish lass), Valentine (synth pulp noir detective), Piper (roving troubleseeking reporter), Codsworth (your former house mr handy), and Dogmeat of course.
Technically, Preston should be available, but for some reason he only gives missions, talks when you're handing them in, and won't actually show up a "join me" option when I try talking to him.