I cycled Basle to Rotterdam just a few months ago.
I planned on following Eurovelo 15 all the way and had the route saved on my phone but for maybe 20% of it I made my own way along some B roads as a combinations of missing the signs and getting lost and some not so good surfaces meant it was sometimes easier to just follow a quiet road vaguely parallel to the cycle path.
I am by no means that fit but did it over 10 days doing between 45 to 85 miles a day, I stopped quite often for wee breaks, especially in the cities where I usually had a wander round and had a bite to eat for 30 minutes or so before going on my way again if I was not staying there.
Being really flat if you just take it easy and spin along I found it a good mixture of feeling tired at night but by no means being so beat I couldn’t go out for a beer or a wander round the cities I stayed in, whilst cycling I don’t think I was really struggling that badly once, which is rare for me.
For a lot of the route you are actually cycling alongside the flood defence of the river and the Rhine is not actually visible, it was always a treat for me to cycle along the actual river as the wind was refreshing and it was a bit more stimulating. The mix of farmland, industrial areas, cities, the scenery of the Rhine gorge and the parks you go through made it pretty changeable from day to day.
I found it really enjoyable and there was a real sense of achievement at the end, I had never cycle toured before and was winging it with minimum research beforehand but it was a great experience.