I used to.  It’s still difficult to maintain good position for that length of time, but getting better.  

Where are your feet when you are standing?  Are they pointing straight forward?  If they are rotated in or out, they could be preventing your pelvis from staying in neutral.  

Are you locking your knees out or keeping them soft?  Locked knees, again, tend to push the pelvis out of neutral.

It may also be partially that you aren’t used to your new corrected position yet and it will take time to build up that strength.  I recommend shifting position often and periodically having a quick rest in your paleo chair.  Stretch out whatever is getting tense.  Reset.  All static positions will result in fatigue.