Any general consensus on whether you should build mileage first or your speed first in cycling?

So if i want to ride 100 miles in 4-5 hours, should i build my mileage up and then work on speed, or speed first and then work on mileage? Assuming I already have a good base fitness level.

From what I've seen marathon programs typically build mileage first and then speed. Is this the same for cycling?