Completely depends on how experienced of a cyclist you are, where you were at the end of last season, what you did over the winter, and how "hard" the 30 miles are.

I stopped riding at the beginning of October (maybe even before then) last year (school, new dog, weather, etc). Did nothing (except attempt to train a dog) as far as exercise all winter. Got on my bike last month and just went. Yea, it hurt. Yea, I got dropped (by racers who had trained all winter...). But, yes, I finished 30 miles without falling over, bonking, or dying. Two weeks (give or take) of solid riding and gym time and I'm getting just about back where I was last year, at least as far as endurance.

But at the end of last season I was capable of doing unplanned centuries with a day's notice, hanging with the "intermediate" group (16-18mph average) for 65 miles, and 30 miles at 15 mph was an extremely "easy" "recovery" day or a mid-week, before work jaunt. If 30 miles was difficult for you before it certainly won't be any easier now, but if it was a wimpy little "get out and spin" ride then it should be no problem. In the middle, well I wouldn't say it's the end of the world so long as you are physically capable of doing it. Just listen to your body, more than muscle soreness or prolonged muscle soreness=back off and then start back slower. Won't ruin your season, if that's what you are worried about.

But you HAVE to hydrate and eat. Otherwise it doesn't matter how in shape you are, you will bonk, and then you will get road rash, and then you'll get a trip to the ER for IV fluids....NO FUN.