I did it in 2011, and it is challenging for two reasons.
1 - as some of the chaps have already said; the variety of surface, wind, and inclines make it a pretty challenging course.
2 - training during December. I found keeping to a plan ahead of Christmas was almost impossible last time. My wafer-thin willpower crumbled time and again, leaving me underprepared and woefully inadequate. I think I finished in 4.35.
I'm toying with the idea of entering again, but there is absolutely no reason why point 2 won't happen again.