This will be my first ever road race. In fact, last week I did my first ever time on a treadmill - 10 minutes at a leisurely jogging pace, and my head did not fall off, nor my spine collapse, as I feared. Also, I reckon if I can keep my anaerobic shit going for 90 minutes without a break then I can handle 5K, which should take me about half an hour tops* (and nobody will be shouting at me if I take a short break).
* Well, to be fair, I have absolutely no idea since I've not done it before. But seeing as this corresponds to an average speed of 10K per hour for half an hour, it would seem quite possible. In any case (conservatively speaking then) I shouldn't be running for any more than half the time I was grading for on Sunday.