Listen to andyp
You have the right base fitness already because you ride regularly and enjoy rinsing yourself on occasion.
As a mid pack specialist the technical things will make a big difference, and most of them needn't feel like training. Dismounting, carrying, and remounting as fast and as smooth as possible are easy wins, and being prepared (and capable) to get off your bike and sprint for a bit. Who knows if it's going to be a muddy year but if you're single speed be prepared to do some running....
Something I've always failed at is knowing the lowest possible pressure I can get away with and knowing the limits of your tyres. Obviously that depends on conditions but riding and ragging your race bike around teaches you these things. Practicing off camber corners and hitting ruts helps too.
I've only ever found course crowding to be an issue on lap 1 or 2. Ultimately if you race more than once or twice your gridding roughly matches your capability. The bigger issue for me has been to learn what I can sustain for lap 1 and 2 without harming lap 6 or 7. Going so deep into the red means you lose concentration and judgement which almost always ends up costing you.
The best bit about cross is going full tilt off road on challenging terrain and having a bit of a scrap without worrying about getting killed by a horse or shot by a dog walker.