Rats! I wanted to give you some good advice, but got busy at work and didn't get it done, and now I see that some of the others have beat me to it!
Welcome to a really great place for bikers on the net. You can learn a lot here, as well as add to strings as you gain experience.
I am 52, and started riding a bike and commuting in April of this year. Now, I've added little detours to my commute to stretch it from 7 miles to 10 to 12 miles. You will be surprised at how quickly you adapt to the ride! I believe that listening to your own body will be a good guide for you. Best of luck to you, and be sure and stop back here often!