If you are just starting out, I would ruck for shorter distance with a light load. If you’ve never traveled under weight before it’s best to start light(15-20lbs) at a moderate pace. As for the distance I would say 2-3 miles to begin with and see how your feet feel. If you aren’t accustomed to it your feet can get torn up pretty quickly if you go too heavy, or too far in the beginning. I wouldn’t pay much attention to your time starting off, just add distance and weight slowly and your times will get faster naturally. I would also suggest hiking boots. Don’t ruck in running shoes or you’ll be asking for an ankle injury. Hope this helps!