This caught my interest and after digging a bit deep into this, I found out that there is a much shorter abandoned road and also a walking trail by the side of the river that leads from Punakha to Wangdi Phodrang.
This route takes you from Punakha Dzong to the second longest suspension bridge in Bhutan and then to the Punakha countryside that is covered with golden yellow rice fields. On the other side of the trail is the azure waters of the Mochu and Pochu rivers that join at the Punakha Dzong.
The waters of this river is so clean that you can see a lot of migratory birds on this trail and those include the Ruddy Shelducks, Cormorants, Stilts and more. Also, on the terrestrial side, there are many interesting and colourful Himalayan birds. If you are a birder, then this trail should be on your must do list in Bhutan.
Even if you are not a birder, the landscape is full of mountains, a fast flowing river and lots of golden yellow fields with traditional Bhutanese houses in the middle. This makes for some great sights and thus presents great photography opportunities.
In terms of distance, I think the trail is about 12 kms with a little bit of ups and downs, but mostly flat. If you take it easy, stop for a lot of photos and breaks, then you should be able to walk between these two rich valleys within 3 hours and if you are in a hurry, you can do it in 90 minutes. If time is not of essence and you wish to experience the Bhutanese countryside with its greenery, birds and colourful people, then this hike is very much recommended.