Training with a pack is a critical aspect of preparing for stage races. As previously mentioned running with a pack impacts your speed and gait and one needs to get use to how it rides on your body. Pack training should be ramped up carefully and practiced in reasonable moderation in order to prevent injuries. Think of it like weight training. Do you go into the gym and do all sets and reps at you maximum weight? Then do you do that day after day? Of course not, and one should not practice pack training the same way.
With regards to weight many find that after ramping up, training at 50%-75% of your anticipated pack weight (less water) is sufficient. Your pack is at its maximum only at the start and will daily get lighter. Yes, some believe in training at over 100% with the belief that it will be”easier” when you start however your risk of injury goes up even more. Additionally your speed will be reduced further as well as your stride length.
One method of measuring your pack training efforts is to multiply the weight of your pack (less any water) by the miles you run with it. For example, if your pack weighs 10 lbs (4.5 kg) and you run with it for 10 miles (16.1 km) you have 100 lb-miles (73 kg-km). As an aside phone books, can goods wrapped in towels, beans or rice make good ballast that is fairly adjustable.
So, how much pack training should you do? That depends on many things including your ambitions. However with any training there are diminishing returns once one is above certain levels. Using the previously mentioned measure of lb-miles most stage races total up to around 2,400 lb-miles (1,760 kg-km). Pack runs totaling 3,000-4,000 lb-miles (2,200-2,900 kg-km) is a good goal for most people. Spreading this out over three to four months is wise and gives your body time to recover and strengthen. Starting pack training earlier than three or four months out most likely will not help that much.
Do not forget that walking with your pack is just as important as running! So are hills and other terrain features you may be racing in (cross country, sand, etc.). It is also OK to slowdown while training with your pack. Speed workouts are better done during other times. Also consider participating in trail marathons in a totally self supported manner (including carrying all water). You will get some looks but you will be fast through the aid stations!
For a peak training week consider five or six back-to-back runs with you pack weight starting at 100% and reducing weight in equal amounts until the last run it is at your anticipated ending weight. This will simulate a stage race quite well. Length of these runs can be some fraction of the expected race distances. The fraction is up to you and your physical abilities.
Finally, back off from the pack runs two or so weeks from the start of your event. You should be good to go by then and any other hard training will not improve your position much but puts you at risk for race ending injury.