McLean was part of the team that were beaten by New Zealand in the World Cup final in September and the Darlington Mowden Park fly-half has vowed to build on Spencer's successful reign.
"To be asked to captain your country is a fantastic honour and when Gary called to ask me I didn't have to think twice about it," said McLean.
"You can't turn down something that you have dreamed about but it's scary at the same time because Catherine did such a good job during her time.
"She was a great captain to play for and I won't have to change too much from her time in charge.
"I enjoyed my time as Catherine's vice-captain and learned a lot from her but didn't expect this at all as I thought she would be carrying on after the World Cup.
"This team doesn't need a lot of guidance anyway - there's a lot of leaders in the squad and that will make my job easier."
McLean, a trainee teacher at Bexhill Primary School in Sunderland, has won 38 caps and will lead an experimental squad into their opening Six Nations clash with Wales on February 6.
Regulars Fi Pocock, Alice Richardson, Claire Allen and Charlotte Barras are all injured while Amber Penrith and Sarah Beale are taking a break from the game.
"It's a really exciting time for the team with the amount of youngsters we've got coming into the squad who are desperate to impress," added McLean.
"We've got a lot to build on from what I've seen and I can't wait to see how we progress in the next year or two.
"We've got a baptism of fire with the game in Wales in a couple of weeks as that's always a difficult place to go to but we will be ready."