And miss easy questions or even cannot grasp what it says

How can I increase/keep endurance without loosing/decreasing concentration?
Are there any way to practice?
Or I will become accustomed to it naturally if I do a lot of 5 section practices?
And...my neck hurts too...
What helped you to refresh during the 10-15minutes break or even between sections to concentrate on next section?
Appreciate any advice!