Important: do not take the "original trail" which is on the left hand side of the river as you go/look up! Unless you want to hold on for dear life, on slabs hanging above the river (I did it so you don't have to)! Take the new 4x4 path on the right (see last two pictures).
Langtang is the closest valley to Kathmandu! Just a 5/6hrs bumpy jeep ride away (that is short by nepali standards).
I had 3 unplanned extra days in Ktm and decided to go for it: afternoon jeep - got there in the night - left early (7am) - ran all day - slept a few hours and got a morning keep back to Kathmandu.
The first 12-15km are jungle single track. Sometimes hard to fall into a pace as it is constant up and down big rocks or rock stairs. It is also a shady valley - so chilly.Although I was wearing shorts, I had long sleeve for most of this part.
Higher up you get into yak meadows and then rolling terrain, at altitude. There are small villages to resupply - do try the fresh sea buckthorn juice - gave me wings for a part of the descent. Easier to run but the altitude can get to you and definitely slows you down. There is a cheese factory at the monastery!
The last part felt gruelling, fake flat uphill! I could see the end but took ages to reach it. Did the uphill in just under 7hrs (6h52). Downhill was fun and runnable until it got dark and I hit the jungle and my torch got into safe mode (very low battery). I have a really good one so the safe mode lasted for over an hour but I was sweating at the thought of being in the jungle in pitch black!
What is really cool is there are variations one can do (adding Gosaikunda lakes or run all the way to / from Kathmandu). You can choose how much you want to suffer.