But the 66-year-old is relishing the opportunity to beat the All Blacks at home once again, having been part of the victorious 1971 touring party.
And Spencer added he was honoured to become only the third Englishman to manage the Lions, following in the footsteps of Fran Cotton and Bill Beaumont.
"They say that the hardest steel is forged in the hottest fire and this is the hottest fire in terms of rugby," Spencer told the Craven Herald.
"I am not having sleepless nights yet but I expect I will do eventually.
"It is the ultimate test as the All Blacks have been at the sharp end of the world game for 50 years, and to have the opportunity to beat them on their own turf is something that I look forward to.
"I am aware of the tremendous honour I have been given, having played for the Lions in the past, but also of the size of the task ahead.
"It is something that I have always wanted to do but never thought that I would get the opportunity."