I have the salamon Xt wings 10+3 and have used it in a half and in a marathon (the half was to try it out and the marathon was an offroad one) I found it to be pretty comfortable for the half but but for the marathon with a full litre water bladder in I found that the bladder dragged down the back of the pack inside and it meant the pack rubbed a little bit where it folded. However i was using a short bladder that didnt touch the bottom of the pack so this may have been the problem. it is more comfortable than a rucksack or the camelpak i have but also where you have the extra fabric across your chest it does get a bit warm.
My suggestion is. (and sorry if you have done all this before!)
have a proper rest for a week or 2, just maybe walk. let things settle and recover
increase your fitness for the next few weeks using low impact eg bike and swim only.
get a gait analysis and check you are running in the right trainers.
I know it sounds like a rediculous backward step but start again using a beginners training program, even go as far back as a couch to 5k to retrain the muscles and not overdo it. (i have done this myself following injury!)
(not sure what your age is buti found Bruce Tulloh's running over 40 book to be excellent to follow for training programs)
take up yoga for flexibility
visit a different physio or better still an Osteo as they are a little more holistic. many running injuries come from problems in other areas.
check your running style to make sure you are not overstriding, hunching shoulders, leaning forward from the hips, etc etc
go and see a physio or osteo, rather than just the doc. they should be able to remedy with massage and prescribe exercises. I hesitate to suggest anything myself as it depends on what is causing the sciateca but often with runners it is the piriformis pressing on the nerve. google 'piriformis stretch' if you want to give it a go but i would encourage you to at least take one visit to a specialist.
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.