Good advice here in general; several remarks:
1 3:00 was on the optimistic side but 82 @ 20km indicates that if you raced a Half, you could probably hit at least 1:25 (as noted or at least implied by several remarks above).
2 Not only should you negative split, but the first 5 km should be a bit conservative beyond that. You need to average 21:20 per 5 km. Start at 22+ for the first 5, then 22 for the next one or even two before going to 21:30, so that you hit 20km closer to at 87 rather than at 82. This will allow you to hit 21 for the next several and if you are capable of running 3:00, you will hit them to the end.
3 I liked to do some of my long runs at marathon goal pace (except a slower beginning and ending), and at least one where I was going faster for at least 25km and usually 30km after a warm up. That way my body was dealing with the type of 'running low on fuel' issues that it faces in a race. I sometimes did this in another marathon, where the setting was similar and there was traffic control and water etc. One way to do this is running a half by running a good 5km warmup and then warming down a bit after and not worrying about racing it as fast as possible, just at 95%. Also, I would ease a bit before and after this one hard workout.
4 During the last several weeks it is easy to train harder than you need to. You are more likely to work too hard the last two weeks then not hard enough. I think that malmo has referred to this has having already put the hay in the barn with several weeks to go. Sometimes this leads to feeling slightly sluggish the first 5-10km of the race, but that fits with the 'optimal' plan of slowly accelerating first the first third.
5 You might have been ready to run 3:00 but it was not the right day. Once things were out of reach, you had not much motivation to reverse things (which is ok), but which accounts for those 25s being 25 rather than maybe 23 which your body might have been ready for. If you had run 3:05 you would have been a little happier but you might have been there in terms of preparation.
6 One advantage of negative splitting is that the race is not 'hard' for most of the way (i.e., you are at least 10% slower pace than 10km PR pace that it does not hurt much the first 205-30 km. Then, as it gets harder, you are passing people rather than being passed and this is much easier psychologically that it is much easier to squeeze everything out those last 10 km.