Actually we chose to not use route 13 through Lao, but we went into Thailand and followed the Mekong southward there. It is part of our route along the Mekong river
from the Tibetan plateau in northern Yunnan to the sea. We had 2 reasons why to choose the Thai option:
first: our Lao-visa were about to expire after 30 days in the northern
part of the country, second: we were told by
people who had experience with this stretch that the Thai side of the
river is nicer to ride than route 13 on the Lao side. The latter is
even more boring and there are less towns along the route.

787 kms along the Mekong river, flat country, apart from app. km 620 -680, where there are 2 climbs of altogether 456 vertical meters. Not very interesting route, mainly quiet main road and little activity. We suffered from and enjoyed a hard northeastern wind. Enough accommodations on the route. Highlights: the Indochina market in Mukdahan and especially Wat Pra That Phanom. The latter makes the trip worth while. Also Khong Chiam is a lay back little town with some nice guest houses, and the National park Pha Thaem just north of it. Here one can see prehistoric rock paintings. Some 30 kms after Khong Chiam is the border crossing at Chong Mek, where you can get a Lao visa on arrival, and again 40 kms further is the first big stop: Pakse. Plenty of accommodations and other facilities there.