“a hard thing.” The “hard thing” does not refer to it being hard for God to give a double portion of spirit to Elisha. God can give as much holy spirit to a person as He wants, and it was not hard for Him to give Elisha a double portion of the spirit that was upon Elijah.
The “hard thing” Elijah referred to was how “hard” it is to carry the responsibility of walking by the spirit. Having the gift of holy spirit comes with responsibility. A person who has the spirit is responsible before God to walk by the spirit—walk by revelation—and manifest the power of God. Often what God told His prophets to say or do was difficult or heart-wrenching. That is why the word of God to a prophet was often called a “burden” (see commentary on Malachi 1:1).
Many of the prophets were persecuted, and the tasks very challenging. For example, it could not have been easy for Elijah to tell the king of Israel that there would be no rain or dew (1 Kings 17:1), because that was very hard on the people of Israel, and Elijah loved the people, but was obeying God. By telling Elisha that he was asking for a hard thing, Elijah was warning Elisha that he was asking for something that would be mentally, emotionally, and sometimes physically difficult. It had been difficult for Elijah to walk as a prophet and obey God, and if Elisha had a double portion of the spirit on Elijah, then it would be even harder for him.
We should also reflect upon the fact that Jesus was given the spirit “without measure” (John 3:34). Jesus had a huge responsibility upon his shoulders to walk by the spirit and obey God, and in doing so set the standard for all mankind to follow.