Sorry to hear that you reckon it wasn't a good race, but like you say all DNFs & 'bad' races give you positives in the long run.
I've seen a few friends miss their marathon goals recently (from 2:38 runners to 4hr runners) ... and realise how much expectation is heaped on an event which is quite long and usually requires everything going to plan over 26 miles to achieve your goal time. Hope you enjoy your training and the next one goes OK. (I think I prefer ultras because often you can focus on the goal of finishing rather than, like you say, "sprinting for 3 hours"

)