It doesn't matter which distance you choose from Sprint to Ironman, you'll spend the majority of your race time on the bike. In fact, the longer the distance, the greater the proportion of the time you'll be pedalling. For serial non-bikers there are Aquathlons, but for the rest of us biking represents the safest way to build base fitness (and the easiest outlet four any gear-head tendancies). Your bike doesn't have to be expensive but it's vital it fits properly. Two good sources of fitting information are slowtwitch and cyclingnews, though their resident experts differ on the details- do your research and get comfy! In the mean time, find out more about:
Sunday Rides: Great training rides in good company
Club Time Trials: Quality intensive training