I have been a proponent of weighted packs for years; but over the last couple of years I have migrated more towards the easy-of-use of weight vests.
My wife added a 44 lb. vest as a Christmas present this year and I love it. I also add 6 lb. ankle weights sometimes, but would suggest caution if one decides to adds these. They can alter your gait quiet a bit and can lead to injury.
I do controlled runs, mountain trail force marches, or just long walks around my neighborhood. I pretty much incorporate this, along with regular weight training and running through out the year now. Even a quarter to half mile jog added to your hike, can get your heart rate UP, UP, UP. Again, start slow and work your way up and phase this type training in slowly but surely.
At 54; it is much easier for me to transition from general fitness to getting fit for upcoming elk hunts at high elevation. With the very cold temperatures we are getting right now, weighted packs are also perfect for short/slow jogs and double time pace workouts, because you can get your heart rate up pretty quickly. It's 9 degrees here in Central AR this morning and I will probably try to get a 30-minute trail hike in today. This type routine can go a long ways towards keeping your fitness level on par during the off season.
This stuff pays off big dividends in overall fitness and injury prevention, come time to hit the big woods on that dream hunt! Salute!!