Of course you will want to focus most of your training on cycling. Nothing will prepare you better than actually riding your bike. It is important to experience the feel of the road, the bumps, the wind, the traffic, stopping, mounting and dismounting. It’s also the best way to toughen and train that posterior region. There is no cross training that will help condition that area like time in the saddle.
With that said, cross training definitely has its benefits and it is recommended that you introduce variety to your total training program. There may be times that the weather won’t allow you out on the road to train or quite frankly, you just need a change of scenery. In those cases choose an activity that is aerobic (gets your heart rate up). Most importantly make it something different that you’ll enjoy.
Cross training examples:
Swimming, running or jogging, walking, hiking, calisthenics, jumping rope, stair climbing, spin class, aerobic classes, dance class, roller blading