totally depends what sort of race you're having.
In my experience, when I know I am on for a good time after about halfway, then I'll give it everything for the remainder. Likewise if there's someone I've been trading places with during the course of a race I might give it an extra spurt at the end to try and get in ahead of them.
But if I've had a poor first half start or if it's very slow conditions or whatever, I'll just trot in and save myself the aggro.
At Borrowdale last year I'd possibly overtrained, with the result that I got to Scafell Pike about 15-20 mins after I'd been expecting, and in some discomfort due to a weak back.
I had absolutely no motivation to complete the rest of the race 'as quickly as possible', didn't care in the slightest. Just wanted to get home.
Most of the time though I have tried my best - often not good enough - in the last mile or two not to give my place away to anyone behind me.