I only got into cycling on a regular basis about a month and a half ago. I'm 6'1" 280ish, and was able to push to about 5 miles. I stayed at this distance for a couple of weeks, and then started bumping it up gradually. Yesterday I did my longest ride, 14.6 miles. I'm sure you've heard this before, but don't push yourself too hard at first, and especially with the heat we're having lately. Take a day off to recover and let your body recoup.
BTW, welcome to the forum, from another Jersey guy. I'm just discovering some of the cool places to ride around here, did the D&R Canal yesterday from Washington's Crossing to Lambertville. It's nice and flat and good shade from trees.