How to increase my running endurance?

Usually I run two miles a day (9 min each) but take a five minute break between them. I recently started this cross country workout every other day (I don't run the 2 miles on these days) where I run on an incline which increases and decreases throughout the workout for 30 minutes. I jog/run most of the time but I do walk some of it (maybe 10 minutes total out of the 30minutes). Will this help my endurance with running? I really need to be able to run a mile and a half nonstop.
How to increase my running endurance?
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