The opposite for me. Prior to my first marathon my longest training run was 15 miles, untimed but comfortable. Timed in at 3:22:15 (or was it 3:15:22? I have to look it up now!) in the race. My best half timed in training was 1:32, but my best race was 1:20:02. Of course, I was just a kid of 39 then...
My fastest marathon (2:48:45) was also my easiest; just felt good the whole way but stuck to my planned pace to break 2:50 (Boston qualifying standard then) instead of picking it up the last 3 or 4 miles.
A lot had to do with the weather. I have never run an official half in under 80 degrees. Makes a huge difference, almost 10 minutes, to my training PR in 60 degree air. I quit running that kind of distance. I decided at some point that this intense cardio workouts are best on your heart to limit to an hour so I am sticking with 10k at this point.