One of my training partners did the Distance Challenge this year and raced. He really didn't get rolling with his bike racing until April or so, but he still was able to upgrade from Cat 4 to Cat 3 by August (all points in 2008). If the training load is ok, you can race all year with this, just set expectations appropriately when you're focusing on marathon training. The bike racing will be there for you later, and you can work on specificity a week or two after the marathon.