Hierro, who was previously the Spanish FA's sporting director, took charge of the one-time World Cup winners on Tuesday shortly after his predecessor was sacked following his appointment as Madrid's new boss.
The 50-year-old, however, also admitted at his first press conference that he has no time to make changes to the squad ahead of their tournament opener with Portugal on Friday.
"It is a nice challenge and an exciting one," Hierro told reporters. "The circumstances are what they are and from now on I accept the responsibility bravely, it can't be any other way.
"We have a group of players who have been together for two years to work towards the World Cup and I have been sporting director, so I understand the motivation is huge.
"We cannot alter two years of hard work in the coming days - from now until the Portugal match, we don't have the ability to change much. "The key is to change as little as possible. We cannot change anything in two days."
Spain's match against Portugal is followed by ties with Iran and Morocco.