We can see from the Battalion's war Diary that Harry is marching to the front line. The distances don't sound too demanding in today's terms but it is likely that the men were carrying their normal equipment and rifles all of which would weigh up to 30Kg (66lbs). On top of this, they were not well fed and had just spent 6 months in the Flanders battlefield.
On 23rd, the distance marched was one of the shorter ones, 7 miles, and they were in Billets by 1 pm. That would give Harry the chance to write the letter.
Nov 23rd 1917
Just a line to let you know I’m going on all right. I should be pleased if you could send some powder the next time you write the cake was grand you sent. I hope you get on all right with your fresh job.my address is the same only put
With best love