Rowan, being scared is very much a part of the process; I think anyone who says they weren't isn't being honest with themselves.
I agree with David; now's the time to eat everything you love, and lots of it. You'll quickly lose your sense of taste once radiation starts, and any weight that you may gain between now and then will melt away. Radiation is truly one of the most effective weight loss programs around...thank goodness it's not used for that!
During treatment, work REALLY hard on maintaining your ability to swallow. You'll hear a lot about PEG tubes; they can be really helpful for nutritional needs, but the risk is avoiding swallowing anything because your mouth and throat hurts. Your ability to swallow anything will disappear quickly if you don't work at it.
It will hurt, but it's important. A PEG tube isn't mandatory, although some feel it should be. I didn't have one, perhaps I should have at one point, however since I'd had one post surgery, not going down that road again was enough motivation for me to really work hard at swallowing my nutrition.
Ask questions, let us know how you're doing on your journey
Wayne