From Delft to Rotterdam - Hard?
This obviously depends entirely on your fitness level and your experience on a bike.

I will say that of all typical 15km rides, the ride between Delft and Rotterdam would likely be in the bottom 5% of difficulty, along with most of the rest of the Netherlands. I haven't done that route specifically, but from my time cycling through the Netherlands I can say that your ride is virtually certain to have no hills, cooperative traffic, and well-signed cycle paths. Unless you are grossly out of shape or have a physical condition that makes cycling inherently difficult, this is likely to be one of the easiest 15km rides you could possibly do.