Thanks again for the replies, I'll try to answer everything as best as I can and give enough information.
1. About getting off during the trek:
I have no idea if she would need to get off, I was just saying just in case. So far, Syd has never begun to tire or walk strangely during a ride.
2. Is there a noticable change when he's being ridden?
Apart from his pasterns bending a bit more, no. On gravel he does walk quite carefully but on grass he walks like any other horse, with a spring in his step, carrying his head normally. He has a fast walk, but he's like that in the pasture, too, so I think that's just how he is (all horses walk at different speeds after all)
3. About asking the vet:
I have asked him about Syd's legs last time he got his shots and teeth done, and he said that as long as he's not doing any jumping other than cross poles, he would be fine. We should just keep doing regular riding exercises with him to keep those legs supple.
4. About the amount of riding he's doing:
Yes, my sis is going to gradually build up the amount of riding, so that Syd will be ready. We'll go for rides around the farm to up his fitness and stamina.
I don't know if I've mentioned this, but Syd is getting a lightweight treeless saddle that he's going to wear during the trek. I've found that he seems to go much better without a saddle rather than with one. His saddle is also quite heavy and uncomfortable to ride in, that's why we're getting the lighweight one.
Again, Syd loves being ridden and willingly gallops across the paddock with my sister on his back, bareback. We're not forcing him to do anything he seems uncomfortable with, and my sis doesn't force him to gallop up any hills, he WANTS to do it!
Anyway, I hope I cleared everything up information wise, opinions are welcome.