I need to get up and walk around a bit.
Can you tell me some reasons?
I have been sitting on these stupid chairs for more than 12 hours a day. I need a rest, you can sit down alone.
You should keep your body moving for a better blood flow. Prolong sitting period can lead to health problems.
I need that. The job is boring and I was never interested in sitting for a long period of time. I feel physically sick if I sit for a long time.
In that case, you are right to take many breaks in order to simply walk around, because your health is more important than being slightly more efficient at work.  Given your circumstances, you may want to ask your company to install a standing desk for you, as this is the kind of accommodation that they are required to offer.