If anyone has done this before, I would like to know whether a bicycle ride from Delft to Rotterdam is hard (approx 15km).
I don’t know what to expect since the Netherlands is flat, but still.
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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.
Your only risk for a bad ride is a storm from the south/south east. If there is a storm, you might consider using the train instead of going by bike.
There is a rather direct route, called 'Rotterdamseweg' on the Delft end of the route and it changes name to Delftweg when getting beyond half way.
When you use Google maps, you can get the bike lanes on top of the usual map, this link should show them. If you do not get the bike lanes, select bike as mode of transport and click on showing bike lanes or 'Fietspaden tonen' if you have the Dutch version.
You can also use the cycle nodes, 'knooppunten netwerk' in Dutch, and this site will tell you which numbers to loop out for, allowing you to get the route address to address. The system works with small signs with a number on each corner. You follow that number till you reach it and then you follow the next number on your list. The routes it uses are mostly nicer to use than the direct bike lanes but it might lengthen your trip from 15 to 20 km. Which is still not much.
For most countries maybe a funny instruction, but in the Netherlands this can be a problem. If you set off in commuter time, you may find so many cyclists on the paths that you may feel insecure. In that case, wait a few minutes. Most of the time the cyclists come in bunches, regulated by traffic lights earlier on in their trip, when the bunch has past the path will be almost empty. You can also decide to start a bit later. In summer, some paths are so popular that they are busy all the day, but most of the time they are less full than in the morning peak hour.
While all routes are sign posted, you may want to have a good map, specially to get out of the one city and into the other.