General Johnston agreed to surrender his army on April 26, 1865. Actual paroles issued at Greensboro, Salisbury and nearby points in NC are dated afterwards. Captain Marett would not have started home until he received a parole. That document or something similar should be found in his service file.
Here's Dunbar Rowland's history of this commmand --
Copies of Capt. Marett's actual service file are available here --