I think that this just goes back to motivation and task management in general. Maybe 20 minutes is just too long for you. Try to do 12 minutes. Set a timer and just make it a rule. When the timer goes off, finish your current problem and then get up. If you feel like getting up before the time goes off, just look at how much time is left and tell yourself "Why get up now and feel bad about it? I can get up in eight minutes" (or how ever much time is left on your timer).

I know people talk about needing to build up endurance for the real test, but more than that you need to retain the information first and foremost. That being said, for practice tests try to push through each practice test like it was the real thing, but for practicing don't over-do it. I am planning on doing a practice test every other Sunday to measure progress and in that case I will push through.

Also, another thing that helps me to stay focused is to go to a place where it is much harder to become distracted or less convenient to get up and change tasks...for example, a coffee shop or the library. At home you have too many temptations.

Best of luck

Set a stopwatch - say 25 minutes for 20 math questions - and then race it. The difficulty of the tight deadline will force your mind to concentrate