I did this loop a number of years ago...probably 10 or so years ago now. Still remains one of my favorites I've done. Once you get above tree line the views are incredible, and you stay above the tree line much longer than many of hikes in the Whites, since you go across the ridge. The down side is that the trail can get a bit busy...but that's because it is that good of a hike.
I'm a real bad judge of how hard or easy a trail is...I tend to just go, and worry about how sore I'll be later (not necessarily a recommendation). With that being said, it's definitely a tough hike, but honestly, I think anyone with decent stamina could do it.
If you haven't already, check out the
White Mountain Guide Book from AMC by far the most comprehensive collection and descriptions of trails that I know of. I own a really old edition of the book, and I'm sure there's even a lot more information in the newer one. One awesome thing about it is it has a recommended amount of time to complete a hike. After doing a few of the hikes, you'll find out if you're consistently behind, ahead or pretty much right on what they have, and it becomes much easier to judge trails. It also comes with maps of the areas, which is always a good thing to have on the trail.
Couple ideas of things you could do to get ready for a longer hike: Not sure what your lunch schedule looks like at work, but getting out and walking for half hour mid-day is a great way to get moving, and it's surprising how much it helps stamina...especially if you can add in a couple hills.
I recently got introduced to
www.alltrails.com when I was trying to find some moderate hikes for my trip to Utah this summer. And after a quick search in the Boston area...there are so many trails. Many might be more of a walk than a hike, but with the days getting longer, I'm sure you could find a few short walks in the woods for after work. Maybe even post in the hiking groups you've been following, and see if anyone was in the area and wanted to join.