Fifty pounds isn't too bad if you are on flat terrain and wearing decent footwear. I wouldn't do it in running shoes, use hiking boots if you have a pair. If you want to work up to running with it, start off at a brisk pace, like 15 min. per mile or better. You can also do it as intervals, but with a wider work:rest ratio, such as 1:00 jog(we call it a shuffle), then 4:00 walking(still keeping a decent pace, not slogging along to rest).
If you are doing it once per week, I wouldn't worry too much about stress fractures, as long as you've already been running consistently for a while. If not, then I would get consistent with running 2-3x week first, then add in the ruck runs. Just pay attention to your body and if something hurts(more than soreness), don't do it.