The research says that short, heavy rucks will transfer to lighter, longer rucks but not vice versa. When pressed for time, go heavy, walk fast and keep it under an hour. If you have the time, a mix of both is ideal. How you structure it depends on your schedule. I have had some folks concentrate on heavy rucks with a weight progression, and rucks no longer than 40 minutes (2-4 times per week), but include a progressively longer ruck every 2 weeks with a lighter weight. That worked very well. You can also do some brutal workouts on the treadmill. Try setting the treadmill at 4 mph and 0% grade. Then every minute increase the %grade by 1 until 10% grade, then down by one to 0% grade. This is a 20 min ruck workout that will crush you if loaded appropriately.
Some folks do well with a run and strength training program and a ruck once per week (progressing in either weight, distance or both). We generally always train at the same speed, about 4 mph.