I think if you're going for times rather than just getting round, you've got to do at least one - if not more long run over 20-22. At 18 miles, you've only covered 2/3rds race distance, and it's from that point where the wheels are going to fall off / hit the wall etc... I think it would be relatively easy to kid yourself that you're ready for a marathon on training runs up to 18 miles. 8 miles is long way when you're in unknown territory.