It's been my experience that walking a mile or two a few times a week will really build up your endurance fast.

I'd say if you do that for three or four weeks you'll be ready to hit the trail and do 4-6 miles with a loaded pack without much trouble.

But I agree with Aimless too, start with some day hikes and a light load and see how it works for you.

And welcome back to the trails!