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A lap of Lake Thun sounds leisurely until the hills and road traffic enter the picture. This is a proper ride through Spiez, Thun, the north-shore villages and Interlaken, with plenty of reasons to stop by the water. It is the road cycling circuit, separate from the long-distance walking route around the lake.
Choose a stretch if your group mainly wants a relaxed outing. For the full circuit, check the current route, road conditions and your bike's suitability, then allow several hours with breaks. An e-bike helps on climbs, but does not remove the need to feel comfortable in traffic.