If you want to cycle the whole route then you will need to book a different trip with different company. There are plenty available. Exodus need to make this clearer in the trip notes. The chances are that the group will be a mixed ability group and there will always be an element of compromise. I did spend a bit of time on the bus when I'd have preferred to have been cycling but I soon got over it. This was a minor niggle that did not detract from this holiday being a truly awesome experience. As a group we got on very well and compromised well so it wasn't a problem.
There are plenty of challenges for all ranges of cyclists. You do need to be fit though. If you like pushing yourself hard to see what you are capable of, then this is a trip you should consider.
Take the Tibetan chef option. It saves a lot of hassle. I think that Exodus should charge a bit extra for the trip and include this in the basic price rather than it being an optional extra.
The excess luggage charges seem to have changed recently on the Kathmandu - Lhasa flight. We ended up getting charged $US150 each whether whether or not we had packed carefully or not. If I had known this I would have packed more recklessly! I was expecting to pay about double this amount.
I'm not sure a down sleeping bag and down jacket were that necessary. Any warm coat & a sleeping sheet would have sufficed and freed up a lot of space / weight in my bag.
I was a bit nervous about the altitude before I went as I had previously had some unpleasnt headaches at relatively low altitudes on previous trips. There was no need to worry. This trip is very well structured with careful attention to acclimatisation, so this was not a problem for me. No-one in the group got proper altitude sickness, just a few headaches as you would expect.
Kumar's advice was helpful and very simple: if you find yourself too breathless to talk to the person you are cycling next to, then cycle slower. It worked for us.