Best way to improve endurance?

is interval the best/fastest way to get better endurance? For the last 5-6 months i have pretty much only done strength training, i think i have been out jogging or done interval 5-6 time or so within that time frame.

So i can jogg about 30 min in one go without stopping, and i an run/sprint/jogg for 20-30 min while doing intervall (3X20-30 sec up a hill, jogg 5-10 min before and after).

I feel like my endurance level isn't good enough, and im working on it. But anyone know if interval is the way that is going to give me the fastest results, or should i focus on running with a low intensity?
Best way to improve endurance?
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