Congratulations on your accomplishment. My wife and I returned to bicycling after 20 years (we're in our 60's now) and we're kicking ourselves for missing out on the last 20 years. You will be surprised at how quickly your mileage adds up. Listen to your body. Don't be afraid to take a day off if things hurt (or go short/easy). Check with your LBS and see if they have any early am group rides with a group at your pace; it's a nice change of pace to ride with a group. Keep riding!!