Would depend on exactly how the king and the land were one, of course.
If it depends on his moral character, things would get -- interesting. One would expect that all sorts of wangling goes on as the nobles and commoners alike try to make someone else the butt of the king's justice. That kings would often risk the realms, or perhaps toss the matter aside as superstition and bring havoc.
If it depends on his mood and temper, I suspect the king would always be a mild and placid soul, and would select his heir with the advice and counsel of his nobles, for such equanimity. Fortunately, unlike Russia after Peter the Great, I suspect that such kings would take care to ensure they always had an heir.
If his health, I suspect there would be a puppet king who would reign for some years in the bloom of health, possibly only after having fathered a child, and then abdicate. Real authority would lie in the hands of the Regent. With some grounds, since youth would be an obvious requirement.