I'm not sure from where you got the information's of backing up the extended private key but all you actually need is the seed (12 words), all private keys are generated from it so that's all you need to recover your wallet even after a few years from now. and see this[1] for more info.

[1] http://support.mycelium.com/hc/en-us/articles/207045535-How-do-I-make-a-backup-