if you want to improve endurance, you gotta reach your target heart rate and maintain it for a min. of 30min. without doing this you wont see any gains in cardio. by reaching your target heart rate and maintaining it for extended periods of time will increase the heart's blood pumping capacity which means more oxygen to your muscles. the specific exercise doesn't matter as long as you can reach your target HR and maintain it.
if running is your thing, then go for distance. if not, jump rope for 30-45min, 5 min break, and another 30-45min. i prefer to jump rope personally, as i can do it regardless of weather and whatnot.