Can I do cyclocross if I've never really been very athletic?
Looks like the main question you have is "what should I start doing now to get myself in shape, so that in a year I can race cyclocross?"

Periodization is the Roadmap - Intervals and Hills are the Key

Periodization
The main principle in athletic improvement is interchanging periods of Base Building / Recovery with periods of Strength/Speed/Skill Building.  Where you are looking at starting a race schedule a year from now you have the chance to go through several of these cycles.

For your base building the primary activity is riding a lot, and maybe some cross training.  You can use an indoor trainer or stationary bike, but with the right clothing and equipment you can continue to ride outdoors.  If 30 miles is your longest ride, you may want to set a goal of riding a metric century (62 miles) in 10-12 weeks.  Any beginners plan for riding a century will work as a guide for your base building period, but you would alternate hard and easy days and do progressively longer rides on the weekends.  Normally base building periods last 8-12 weeks.

For your Strength/Speed/Skill Building you add event or goal specific activities - but the two primary activities are intervals and hill climbing.  Doing a search here on 'intervals' will give you some direction, but basically within a given ride of say 30 miles, you would intersperse periods of high intensity riding with brief recoveries. Doing so trains you to ride efficiently at a higher level of effort.  Climbing hills works the same way and builds muscle strength.  Typically you follow a base period with 2-3 build periods each lasting 3-4 weeks with a rest week in between.

After doing a couple of build periods you go into another base/recovery period.  So, if your goal is to start riding races in October of next year, you can build a calendar backwards from your first race and build in several build/recovery cycles.



For good books to get training ideas look at Book Recommendation: Training

For building a base or strength while commuting look at How to make the most out of daily commute?