Gary - love the photos - mind if I ask how you planned and arranged the riding logistics? Probably not an option for me, but looks great so I'd be interested to learn more.
Bit of lucky timing really - while I was procastinating about if/when to go and a trip I had an eye on with KE Adventure dropped off their schedule. So faced with a choice of pitching up in Kathmandu and sorting something out on arrival or arranging something in advance I did a bit of searching and turned up an interesting sounding trip around the right dates with http://www.nepalmountainbike.com.
From what we were told, we lucked out with our guide (a french ex-pat) and if planning biking you would want to ask plenty of questions about the type of riding, guiding experience etc. to make sure you get what you want. But you can find people offering trips varying in length from 1 day upwards so don't write the idea off. It can be as easy as riding out of Kathmandu to the valley rim, and it was an interesting way to see bits of the city away from the chaos of the centre and the touristy parts.
So nice laid back arrangement for riding sorted out and I just booked a flight. We were well looked after from arrival with just a little free time to mooch around Kathmandu doing my own thing before flying out.
It would be pretty easy to pitch up without anything planned and get some stuff lined up. It would just depend on your general feeling about that independent approach and how much time you have available. If you have an idea what you want to do you can track down most of the well regarded agencies online.
If you go trekking, yes you are heading somewhere remote but if my experience of Mustang is representative then you are still very much in the realm of the tourism industry so it is still very friendly, plenty of places to eat and sleep.
Kathmandu is a crazy chaotic place but fun to explore for a few days. Biking around was actually a good way to see it, 5" of suspension quite good for some of the roads. Traffic is like a study in pedestrians, bikes, cars, etc. being left to their own devices but on the whole seems to work!
Pokhara is more laid back around lakeside though when you get up on the hills you suddenly see it is actually much bigger than you imagined. Even coming from Nepal, the 30 minute flight into Mustang felt like being dropped into a different world.