I'd go with a two-part series in the vein of The Greatest Knight and The Scarlet Lion- both books on the Marshal's life, split into two parts, by Elizabeth Chadwick.
Young(er) Marshal: Liam McIntyre (played Spartacus, seasons 2-3); Luke Evans is also great choice, or Henry Cavill, or Nikolaj Coster-Waldau (Jaime Lannister on GoT)
Old(er) Marshal: Liam Neeson (he played a similar character as the Baron d'Ibelin in Kingdom of Heaven) - best choice, Crowe would be a great second
In the research materials I've gone over in the past all personify the Marshal as imposing in both physical and charismatic attributes so I purposely chose actors who can bring both to the big screen.