I wouldn't worry about changing your diet this early in the game. I was a bit older than OP when I got back on the bike. After a couple weeks, I started riding with my daughters down to a bookstore 5 miles away for iced coffee. My wife would drive down and meet us, then we'd load up the minivan and drive home. Half way through the summer, I started riding back home, and by the end of the summer, the girls would usually ride with me.
It took me 5-6 years before I rode my first metric century (62 miles). Another 5 years, and I rode across the U.S - but not in one day!
Point is, you need to give yourself time to build up stamina and cycling muscles. I'd suggest backing down some; if you're wiped after 30 miles, start riding 15-20 miles a day, 3-4 days a week. After a month of that, try 30 miles on a weekend. If you start eating a bunch of junk now, you're more likely to put on extra weight and suffer indigestion than be able to ride 50 miles next week.