μάραθρον
From LSJ
ἃ γὰρ δεῖ μαθόντας ποιεῖν, ταῦτα ποιοῦντες μανθάνομεν → what we have to learn to do we learn by doing
English (LSJ)
[μᾰ], τό, less Att. form of μάραθον, Alex.127.5, PTeb.116.43 (ii B.C.), UPZ89.9 (ii B.C.), Dsc.3.70 codd., Gal.12.67, Hld.6.14.