Welcome, new people!
Freddo, sadly you are kidding yourself, according to the clever people who know about these things. Apparently you will be re-stocking your glycogen reserves during the day, thus eliminating one of the key elements of the long run - learning to burn fat rather than glycogen. Still, it's a very useful session.
Neil, yes, you're in the 3:45 ballpark. The last miles depend on preparation, pacing and one or two other things, but are likely to involve pain and extreme willpower. If you've gone off too fast, or under-trained, or not fuelled/watered yourself adequately before and during the race, you will essentially grind to a horrible, painful walk. The good news is that it only lasts an hour or two (or three?)!