Ideas for a workshop to do the user journey [on hold]

We have been asked to come up with a plan for a workshop with stakeholders in which the output would be the understanding of the user journey, the user pain points and brainstorm opportunities for the product.

Unfortunately we won't be able to have access to users so the idea would be to do this with stakeholders and their assumptions.

Could you suggest any ideas for activities that we could do in the workshop? If you have any article where they show the "step by step" or even agenda also that would be great.

Finally, how much time a workshop could take approximately? What should be our time estimation?

Thank you so much in advance,