I think you need to break your goal down into manageable bites. If your goal is to complete a LD ride (to start) there are many smaller milestones you need to achieve on your way, and breaking them down can make it all seem much more manageable. In short, yes, you could do an endurance ride by the end of next summer. Whether that timeline will work out best for you and your horses is an open question and depends on so many things.
There are some good sites that have info for beginners, including this site, which was a fabulous resource for me when I was still in the thinking about it/planning phase: Endurance Riding
I also really like the book Endurance 101.
I might suggest that step 1 will be getting your horses to trail-horse level before worrying too much about the endurance side of the house. So have them well trained (w/t/c and whoa at least), used to those scary blowing leaves, logs, crossing water, mud, cars, bikes, four-wheelers, kids, dogs, cows, horse trailers (loading and unloading well too) and crazy human things like BBQs and picnic umbrellas.
Once they're okay with that stuff out on the trail (most of the time at least, we all have 'off' days), then start focusing on adding more time and miles in the saddle each ride. Unless you're in fabulous shape already, this conditioning will be huge for you too. Good luck! Their age should be no issue at all, they're pretty much in the prime of their life to go for whatever you want them to, so just make sure to be reasonable and not push too hard too fast and overcondition or injure them. The heart and lungs can adapt to exercise in a couple of weeks, the muscles in 4-6 weeks, but the bones, tendons, and ligaments take months, and are the most likely to cause permanent issues if you try to do too much too quickly.
Please share pictures of your boys when you get them home!