In many of my previous posts I have stated, that a reasonable estimate of a maximum carry load that a relatively physically fit man can carry for long distances - is about ~23kg. However, I think it would be best for me to elaborate a little bit and provide you with a sources of such information.
The following quote is from a military bulletin, that is called "Comparative studies of the field equipment of the foot soldier of the french and foreign armies" by Commandant LAVISSE (January 1, 1906, Stanford):
What is the maximum limit which the weight of the foot soldier should not exceed?
Quite interesting experiments in this line have been made in Germany.
In the month of April, 1894, on the formal invitation of the military authorities, the medical students of the Frederick-William Institute put on the uniform, carried the campaign load of the German foot soldier, and executed a series of marches with the idea of studying the limit of resistance of the human body amid the fatigues of war.
The marches undertaken by these medical students varied from 24 to 75 kilometers. They were executed under varying conditions of temperature, and with loads of from 22 to 31 kilograms.
The conclusions drawn from this series of experiments were the following: When the load of the foot soldier is moderate, not exceeding 22 kilograms, a march of from 25 to 28 kilometers, under mean temperature conditions, has no ill effect on the health of the soldier, and he maintains his muscular activity.
A similar march, but under conditions of great heat, causes a slight disturbance of the organs—abundant perspiration, precipitate beating of the pulse, too rapid respiration, abnormal circulation of the blood. These symptoms are not grave, disappear after a few hours of rest, and do not diminish in any way the power of resistance to fatigue during the following days.
A weight of 27 kilograms carried during marches of from 22 to 28 kilometers, executed in favorable weather, is also easily borne by the soldier, and does no injury to his health. But this same load carried during very warm weather causes in the human body perturbations, the injurious influence of which makes itself felt afterwards.
The weight of 27 kilogram must then be considered as the maximum with which the average soldier can be loaded during marches of from 25 to 28 kilometers executed in summer.
As to the load of 31 kilograms, it has a bad effect on the physique of the man, even during average marches and cool temperatures.
In the matter of training to support the campaign load, it may be remarked that a light weight of 22 kilograms is no longer fatiguing at the end of a few days of practice, while that of 31 kilograms never ceases, even after a long series of marches, to cause a gradual enfeeblement of physical endurance.
So, as you can see, that even though the Kohathites were to carry their burden upon their shoulders, a ~22-23kg weight (which is considerate a moderate weight) would be highly justified and a reasonable one. And even if we would take in the account all specific details about the transportation of their burden, including the fact that they would have to carry their burden under high temperature conditions of the desert, such carry weight would still be reasonable and justified.
Here are some other books that you might want to check out, that mention similar numbers, as well as provide some additional information on the subject:
And here is a relatively modern book on the subject (2005):