As I've said in my email, I have no idea how fast you can travel in the Indian plains. When I was there in 88-89 we hardly ever exceeded 60 km/h due to traffic and bad roads. Times change...
However, I was able to make good distance in the Terai, i.e. Mahendranagar Eastwards. However, it is monsoon now... I suggest you do not go through Pokhara, as the second half of the road is congested, all the trucks and buses take that road. Continue until South of Ktm, then head North over the Tribhuvan Rajpath pass. There will be little traffic, the road is too difficult for heavy vehicles. It was sealed back then.
I just checked my GPS tracks and I left Pokhara about 11 and arrived in Ktm about 14.30. Then it took at least another hour trying to find the guesthouse.
If you are going to Thamel head North on the ring road, not through the centre. It's the first big intersection with lights if you are coming from Pokhara.