Prolonged standing

Hi.

I transitioned to barefoot a year ago. started with VFF which I now use mostly for walking\running, and have only minimalists shoes for everyday.

At the begging my feet sore frequently. I took my time and made the transition gradually. after few months I felt pain only after long walking (nature hiking for couple of hours).

Now, I thought I am completely used to barefoot, and haven't felt pain for months. Until I started a course about some archeological and religious sites, which require being on foot for hours - mostly standing. I got fatigued quickly from the standing, and at the end of every day of the course my feet is very sore.

Is it normal that one that is regular to walk more than 10K steps each day barefoot will find it very difficult just standing for 20 minutes?
And what can I do to overcome this? It has been more than 5 tour days and I dont feel it getting better.

BTW I tried one day using my old insole (quite expensive, I liked them before being barefoot), and it helped a lot with the feet fatigue