I re-started cycling in 1988 after years of motorcycling (the worst form of transport because you never need to walk anywhere) and thinking I was fit. I bought a Raleigh Maverick on impulse from a small bike shop in Ripon and took it away, saddle height un-adjusted, tyres not pumped, nothing set up. After three miserable miles heading back towards Summerbridge, about seven miles away, I was absolutely shattered and had bonked. A neighbour passed in her VW Kombi and said later that I looked so tired that she had considered stopping but thought I might feel insulted - I would have gladly accepted a lift! The experience was so depressing that I didn't touch the bike for several weeks. Then I began thinking: "Hang on - people ride bicycles for hundreds of miles, you should be capable of seven at least!" So I re-started and gradually built fitness. I mountain biked more when I settled in Rossendale, surrounded by some of the best mountain biking in Britain but I didn't get properly fit until 21 years after that when I went over to road cycling.
So good luck OP, don't push yourself too hard at first and give your body time to adjust to the new stresses. Stop snacking and drinking beer, walk as much as you can to compliment the cycling and remember that the improvement happens in the couple of days AFTER you have stressed the body, so eat wisely.