If you also add that you appreciate the opportunity for roaming the mountaintops with them, and maybe make a small effort to keep up with them meantime, I would hope they would accept that. Perhaps they find common ground with you when sharing ideas for the forthcoming trips?

I know that in some circles there is a tendency not to accept people who aren't as oriented towards certain kinds of technology. As to these two, I hope their love of sharing the open air with their fellows is stronger than any of that. I also hope their existing long term friendship doesn't get in the way of opening up to extra members surrounding the walks.

Maybe while you are on a walk they will occasionally share conversation on a topic that you can join in on.