Why do you want to train to run with such a heavy pack?
I have a friend who did the army thing- run with boots and bergen- he trained with dog biscuits and I can sort of understand why.
For a MM I would try to reduce the load- buy, borrow etc. It is different to running a few hours for one day and on a path. If you intend doing more than one MM it wuld be worth some investment. If you are doing the MM as a pair I am sure you can reduce the load
I only wigh 50kg but get my pack down to 5kg even when I do solo MMs. That is comfy to run with. I could run with a heavier pack but i would hurt more and perhaps risk more injury due to making me more unstable