Pokhara is about 4 and a bit hours north of Kathmandu according to Google maps.
To get out of Pokhara it is about a 30 minute walk to the main bus stop. If you go just south across the bridge, there is a nice stretch of road with adequate room to pull over (not that really matters in Nepal--people tend to stop wherever they like) and it was from there I got a ride toward Kathmandu in a matter of minutes.
All up, hitch-hiking was pretty easy all the way to Kathmandu, but it was almost all in trucks, and they were all only going relatively short distances. It took me five trucks and a car in all and it was really slow (about 8 hours). That said, the Nepali truck drivers were super friendly, I got bought lunch and bananas and it was an awesome way to see Nepal.