Why does the last mile feel so long?

Why does the last mile feel so long? Is it because in a half marathon when you pass the 12 mile marker there are 1.1 miles still to go and that extra 100 metres messes with my head? Or it might just be that after 12 miles my legs begin to anticipate having a rest and they just slow down!   

Today I have DOMS* because yesterday I ran in the River Thames Half Marathon. We were bathed in glorious sunshine and it was a great event. It was very well organised, with helpful volunteers before and after and supportive marshals and spectators during the race. My blistered foot didn't hurt! It was lovely to chat to some runners, including Dave and Steve, along the route. By the 2 mile marker I had overtaken the 2 hour and the 1 hour 55 minutes' pacers. My plan was to get to 10 miles in 1 hour 30 minutes and then do the final 3.1 miles in my normal parkrun time which is about 25 minutes. So I was pleased to hit 10 miles in 1 hour 24 minutes, feeling strong, but then I faded a bit in the last mile. I was glad to cross the finish line in 1 hour 53 minutes, which meant I shaved a minute off my previous half marathon PB. A fun and enjoyable morning by the river, despite never catching sight of the 1 hour 50 minutes pacer!

* Delayed onset muscle soreness