15 miles at threshold pace will do more for endurance than 30 miles at an endurance or recovery pace whether or not you pair it with another 15 miles at some other time during the day (although it won't tell you as much about fit problems that become uncomfortable after the first hour).

There's a lot of latitude interchanging time and intensity both gaining fitness and spending it. I felt great riding my first century in 5:45 at an all-day pace with just a couple 40-50 mile rides before that and a hard 20-25 miles on my lunch hour three days a week.