About 700 kms.  It took us 9 days.
Day 1: Pakse – Champassak, app. 40 kms. South over rt. 13, after some 35 kms leave the 13 and turn left to the ferries, one for cars and one for 2-wheelers. Both ramshckle makeshift boats, ours was made of 2 cano-like boats, attached to each other by a wooden platform of 2.5 x2.5 mtrs, thus a catamaran. 30.000 kip to the other side. Champassak is a sleeping one street town, former royal capital. Wat Phou, 1o kms south is worth while visiting.
Day 2: Chanpassak – Khong Island. 106 kms. Back to rt. 13 and near the island another ferry. This time one cano-like boat. Nice and quiet island, one of the 4000 in this part of the Mekong. Can be 12 kms wide here in the rainy season. Stayed in Mali guesthouse, belonging to a couple that head fled Lao in 1979, for obvious (political) reasons and that now held the Canadian nationality. Nice place.
Day 3: Khong island – Stung Treng (Cambodia). 100 kms. Back to rt. 13 and south to the border (40 kms). Visa on arrival, 21 USD. The road in Cambodia is good, but very deserted and remote. Stung Treng dirty town, where you don’t want to stay.
Day 4: Stung Treng – Kratie. 140 kms. No accommodation in the middle (well, there is a guesthouse, but it is said they don’t have matresses), so we took a bus till somewhere 40 kms before Kratie. There we got off, left the main road and cycled to the riverbank. Nice ride, met some new an old cycling friends and saw some of the river dolphins that live there.
Day 5: Kratie – Chlong. 32 kms. Dirt road along the riverside. Nice, many houses and hello-ing children, also a number of passing cars, so completely dusty.
Day 6: Chlong – Kampong Cham. 87 kms, 32 of which dirt road. A ferry across the mekong after 50 kms. Very nice stretch, hardly any cars. Muslim country.
Day 7: Kampong Cham – Kampong Thnor. 80 kms. After app. 50 kms a 10-kms stretch of very bad dirt road, sharp rocks, difficult to ride. Very remote area. 3 guesthouses just before the bridge, the last one on the left seemed the best. Good restaurant 50 mtrs round the corner to the left.
Day 8: Kampong Thnor – to Kampong Thom. 38 kms, easy.
Day 9: Kampong Thom – Siem Reap. 140 kms. At lunchtime, after 50 kms, we took a bus. The reason was that it was December 31st and we wanted to be in Siem Reap that evening, instead of in Kampong Kdei, in the middle of nowhere. Good choice, we had a lovely night in Pub street. Siem Reap is the base for a visit to the Angkor Wat temple complex and therefore the most touristic place in the country. This particular evening we enjoyed this very much. A nice party atmosphere in the bars and the street, that was full with people and music. Fireworks at twelve.
Angkor Wat is enormous, impressive and amazing. It takes some days to see it. We enjoyed our stay in Siem Reap a lot.