As you head toward Saitama on the Arakawa Cycling Road, you'll come to a point where you need to veer off the riverside. While at first glance, it might seem like you could still climb the Tone River, the Arakawa Cycling Road doesn't allow you to ride along the river for long stretches. If you can skillfully ascend and descend along the levee and riverside, crossing overpasses, it's a very pleasant cycling road.
There are many golf courses nearby, and the view feels more like riding alongside a wide riverbank than peering down at the river.
I also ride at night, but it's generally pitch black. While you can see streetlights in the distance, the ground is uneven in places, so you need to wear a good light and keep your eyes peeled for both the distance and your surroundings.
Also, you won't find any vending machines or other sources of hydration along the cycling road. There are no convenience stores or vending machines until you cross the levee and descend into the city, so make sure to carry plenty of water. You'll encounter a lot of insects, so it's not a good idea to ride with your mouth open. I recommend wearing sunglasses and keeping your mouth closed.