What a lovely idea and how special to have so many joining you. I have hosted several large family gatherings, so here is what I have done;
I would do a group email to everyone, explaining how the 2 days will work. For example, you could let them know the sleeping arrangements, if bedding, towels etc are provided, the catering arrangements for breakfast, lunch and dinner.
Is breakfast provided ? if not, it may be worth asking people to bring their own breakfast stuff, you could say you will provide the basics, such as bread, butter, milk, orange juice, tea and coffee, but that way you won't have to provide everything and they can make sure that they have what they would like to eat.
You could suggest a walk and a pub lunch on one day, no need to think about food that day.
I would also explain that you are providing some wine and beer, but that any donations would also be welcome. you could joke that the nearest shop is x miles away!
Finally I would really think about getting paper table cloths and paper plates in, I think it makes it all so much easier if you only have to wash up cutlery and glasses.
I tend to host the big family gatherings that we have and I have found that people always want to contribute to ease the load. Ours tend to be for about 20 and that is quite a lot if you are doing it all yourself. However, when you spread it out it all becomes much easier, therefore, I would also always ask people to bring a dish or 2, it is amazing how interesting a buffet becomes when you get dishes from different people and it also creates a lovely feeling and something to chat about when people discuss the lovely foods.
Have a wonderful time. Our family gatherings are very special and something that we all love. I am sure that yours will be very special, filled with much love and laughter and the source of many special memories.