My first ride as a "born again" cyclist was about five miles. I thought I was fit so I bought a cheap mountain bike and jumped on it to ride it home, tyres half inflated and seat at the wrong height. After three miles I was exhausted and I was so tired on reaching home that I stuck the bike out the back and didn't touch it for several months. But then it niggled me and I started thinking: "Hang on - people cycle for hundreds of miles, so you should be capable of better than this!"
I was 32 at that time. I'm in my 60th year now and fitter than ever because fitness and stamina go on building. Just remember that as your muscle strength increases you will be putting more stress on skeleton and joints so take it easy and allow the body to build up to meet the demands you place on it.
My advice to a cycling newbie? Take pride in your health and physique; give up smoking, cut down on beer, give up sugar, sugary drinks and sweets, give up margarine and switch to butter, exercise twice a week if you can and above all, allow the body time to recover because it's during the recovery time that you grow muscles.
One simple trick I use to prevent me from snacking in the evenings is to finish my evening meal then go upstairs, change, wash, and clean and floss my teeth. This gives me a psychological "barrier" which helps me to avoid the temptation of that late night biscuit or chocolate.