"Go straight up to five miles."
.i ko ca'u klama sela'u lo minli be li mu
"G straight for five miles."
Saying {lo mu minli} is like "some unspecified number of five of some unspecified (though probably one) miles which may or may not be contiguous and by context probably are not or I would have just said so explicitly". Don't be lazy. <3
Optionally, perhaps:
.i korbi lo minli be li mu lo'o be'o ku ca'uku
"It's at the edge of five miles, straight."
...or some permutation thereof.
Perhaps?:
.i crane do seji'e ja sela'u ja semau lo minli be li mu
"It's straight in front of you, up to/for/after five miles."
Maybe that's more like what you want to say?