Emmy i've not run a long way on sand, only at the end of the Studland Stampede but i will agree with the others, find the firmest sand, usually near the water. and shorten your stride but lift you knees slightly more. dont think it matters if you wear trail or road shoes.
well blisters are caused by friction so something must be rubbing
is the arch perhaps bigger than you are used to? maybe putting more pressure on an area that didnt have it before? is your foot moving around in the shoe at all perhaps they are a tad too wide? when you say anti blister socks are they the 2 layer ones.
how close to exercising do you eat or drink something beforehand? could be that if its an intense session the intestines stop working (blood goes to your muscles) but there is still stuff digesting. could be that your hip flexors are worked hard and because they are close to intestines the sore muscles are transferring the cramping? (after all period pain can cause a similar thing) could be it triggers something like IBS. worth trying running on 'empty' to see if it makes a difference?
I cant add much to what anyone else has said, especially booktrunk! think she has said it all
however i would just say, if you reach a week in the C25k that is too difficult, and you cant manage it, dont be worried about doing the week over, no one will judge! I had to do that several times when i started!