You should have a bench of people you can call on at any time.

You need someone to do your role for some time. or you need to fill a position. You should have a internal and external bench.
Create a internal bench by deleting tasks to your people. Delegation is the greatest way to encourage organizational capability.
Teach your people to do your job by giving them your work.
Coach them on your small responsibility duties. Avoid the temptation to delegate what you don't like. Never delegate tasks where a slip-up would concern your boss. Delegate nearly everything else
Create a external bench through networking.