"Fecimus longum itus sed nunc possumus finem videre"
I would use 'iter' here. 'Iter' neutrum, so it would be 'iter'.
Itus -us, m. So if you use this word, I would say 'itum'. But itus is more 'the act of going', iter is more 'the result of going, the trip'. I would use 'iter'.
So it would be: Longum iter fecimus, sed nunc finem fidere possumus.
It takes a long way to have the ability. So I think the contrast is between the 'longum' and the 'possumus' so I put these to at the beginning and the end to emphasize them.
"Clauso mari, hostium naves non poterunt transportare milites."
I would work with a adverbial clause and with a futurum exactum / perfect:
'Si mare clausum erit, hostium naves non poterunt transportare milites'