ΔE 20.3, loose: different enough that you should pick on purpose. Brown RLM26 is a little darker, much more saturated, cooler than Battle of Marathon.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Brown RLM26 is a little darker, much more saturated, cooler than Battle of Marathon.
The same difference applies the other way round: Battle of Marathon is a little lighter, much more muted, warmer.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Battle of Marathon on the colour wheel See Brown RLM26 on the colour wheel
All matches: Battle of Marathon equivalents · Brown RLM26 equivalents