I am a big fan of responding spontaneously to the Spirit of God and of his gift of administration.
Some think that the only way to be spiritual is to avoid planning. However, far from beneficial this tends to reduce quality and lead to shallowness.
I've heard preachers say "I planned this but God has just given me this instead" and then listened wishing for the planned, prepared, edited talk... crafting a sermon makes a massive difference. Given God is the great former and filler (Genesis 1) it seems to me that use of a diary, advance planning and preparation have the possibility to be very spiritual activities. I need God to meet me in my study not just at the pulpit.
I remember reading of how Stuart Townend plans to lead worship - he looks very spontaneous 'in action' but really is ultra planned with various different plans that he can use depending on how things go. More planning not less gives freedom for spontaneity. A detailed script means I can paraphrase and digress if necessary, knowing with clarity the main thrust of what I want to say and the reasons and arguments that support what I'm saying.
And I think of the great value of big conferences (and small ones) and know that it makes such a difference to the event to be able to book a really good speaker (knowing that such people are in demand... late bookings don't work), and knowing that it serves a speaker (and their family, their work etc) well to be able to prepare thoroughly, seeking God in his study rather than just working at short notice - though I'm amazed at how flexible and servant-hearted some speakers are to short notice invitations.
The other extreme is to be rigid to the plan, unable to flex - whether in response to the work of the Spirit or to the weather, the circumstances etc. Because we planned to isn't always a good reason to execute the plan, but at least if you have a plan you have the option!
Bob Kauflin shares on this subject, Jeff Purswell interviewing CJ Mahaney on this subject