Part of preparation for papers is, actually, building the stamina to keep going.
I say that even though I am in full agreement with brilliotic , in that not everyone is expected to be able to cover the whole paper. I'm just talking about the physical stamina and mental resilience to keep going. It's a skill even the very able need to learn. I would say it;s a skill my own dd finds a bit tricky, even at the age of 14!
You learn that by doing papers and timing them. If you are in the privileged position of working 1:1, you sit with children and talk about how they don't get stuck on questions they get flummoxed by (this eats time, energy and resilience) - you get them to asterisk the question (so they can find it and come back to it) and go on to the next.
Repetition gives them a bodily sense of timing. It's one thing to explain 'you have blah blah amount of time, so spend blah blah per question on average,' it's completely another for children to have internalised what that feels like.
Initially, you can focus on just getting through a paper. Going slowly, taking breaks and having a bit of banana. Then you can try introducing the timed element: 'OK, here's a clock. Keep an eye on it and see how much you can do in the time allowed."
Then you bring the two elements together.
Make the actual maths skills a separate element. If you are teaching time/completion skills, concentrate on that. Look at the actual maths skills - elements of maths the child is finding tricky/hasn't covered - in a separate session if need be. And take your time over that, in a really low-stress way.
The two skills (maths skills and the actual skill of sitting a timed paper) are, I think, separate elements.
It's one of the transferrable skills from 11+ coaching.
For what it's worth, I think that, precisely because it's a skill, and some children will have more practice at it than others, it's one of the things that makes SATs somewhat unreliable as indicators of children's (and school's) attainment. But that's a whole other thread.