Unless your an elite sports person, If you did two 15 miles, with a 10 minute break, of the same intensity as one 30 miles it would make next to no difference, fitness wise, having that break. There is a university study website somewhere that shows the difference with breaks of continuous aerobic exercise (various not just cycling and studies all sorts of people from elite to people just starting) that shows what length of break actually effects the aerobic benefits. Its negligible under 30 minutes if I remember it correctly.