Is there a specific type of workout necessary if you go on bikepacking. I believe I haven't ride a bike 4-5 years and would think bikepacking is extremely different from biking in a park.
Specific to bike packing? Not really unless you're shooting to race and even then my feeling it's genetic. People who are successful racers are just born stallions while some are born donkeys. So I can train really hard but the best I can ever be is a fast donkey.
Anyway, you're just trying to build endurance. Modern techniques to gain fitness involve more high intensity than simple base miles, but for most regular Joes there's really no substitute for just getting your butt on the saddle a lot.
The one thing I might mention is it's not bad cross training to backpack and hike, at least for me. I tend to hike-a-bike quite a bit. Sometimes after a couple of days of grinding it just feels better to walk a few miles pushing the bike. Also lifting and keeping your upper body in shape is important since cycling alone doesn't do that well. If I don't have access to gym (which I normally don't) I find my neck and shoulders get sore quicker.