I think that these 'step' relationships can be fantastic but also very difficult at times, and in my experience the difficulties sometimes arise when the (step) children come on holiday.
You don't know how he would have reacted on this occasion had you been his first wife i.e. the mother of his children. It may be something not to take personally. Rude of him of course.
In my case, we have been together for 15 years and married for five. My husband was a widower with three children, one is disabled and I have voluntarily become her main carer.
But the needs of all his children, now adults, two with their own children, can sometimes (often!) supersede my needs.
I often babysit for my step-grandchildren, far more than my husband does - the children like me, but I know we'll get quite a lot of anxiety from my husband before they come to see us, or before we go to see them.
We have been on a few holidays with his family and his in laws, although my husband won't come on holidays with my brother and sister and their families, etc. Sounds familiar, I think?
Interestingly, I seem to have been accepted by my husbands family and his late wife's family (as far as I can tell, of course) but the people who seem to have a problem with me are one set of in-laws (my stepdaughters MiL and FiL) but thankfully we don't see them more than once a year.
Regarding the extreme walking, is your husband trying to prove that he is still young at heart and fitter than all the younger people? Could you stay behind and get a lovely tea ready for them on their return?
I have just read a book by Susan Forward, Toxic in laws, recommended by someone on another Gransnet thread, and although none of the scenarios nor my issues with in laws (step or otherwise) were specifically addressed by the author, there was a lot of very helpful general advice, particularly about not being a 'victim' in the extended family.
I can't really suggest anything except that in my experience, just 'smile and wave' through this difficult day or two. Hope that once you get home, his behaviour improves. ?