Congratulations. I too am rather new to cycling after years of running. I started riding this spring in preparation for RAGBRAI which is a big annual bike ride across Iowa that involves over 20,000 people each year. I started small and worked my way up. I remember my first 25 mile bike ride and how good that felt. I put in 1,300 miles prior to the event and nearly 500 during the week of the event and it felt great to accomplish something like that. No matter how far you need to ride, you can always break it down into a series of shorter rides with stops. My riding partner and I made it a habit to get up on Saturday and ride 50+ miles to a favorite brew pub and then have my wife collect us. It made for a good training exercise and gave us a reason to keep on pedaling. Keep going, you are doing great!