Pretty place with some scenic views, there is a walking/cycling path cicrling the lake, but it was in a bad condition couple of years ago - more suited to mountain biking with all the tree roots sticking out and asphalt cracks all over the path.
If you stick to the road, however (see Malte's link) you can enjoy a nice twisty ride with short ups and downs.
The cycle path that leads up there from Ome-kaido, parallel to the train track, has many gates. It feels like there's one every 5m. Most weekends it's choc-a-bloc with muggles. I found it very frustrating and definitely not conducive to a workout.
The road at the north of Sayama-ko (northernmost of the two reservoirs) is in very bad condition, as I recall.