A good cadence is something you can always refine later, and could be a good habit to develop soon as it could save some wear and tear on your knees and other joints. However, in terms of fitness - getting on the bike and getting moving is 90% of the battle. Listen to your body, but also make it fun. Push yourself past your comfort level - but don't push yourself to the point of pain. I would hate to look at my bike with the same disdain I do when I looked at our treadmill (I finally sold that, now can't find anywhere to hang my coats). Take time to stop and look at things and enjoy the ride. That is what will keep you going - the love of the ride - not the fitness you get out of it (which will be a pleasant by product).