Many people are confused about eating before a race. Personally I have gone back and forth on this but now I feel that eating -and drinking- less is probably best. Unless you are competing in stage race or multi-day event. It is also best to eat at least 2 hours before the race and if you can't do that, just skip breakfast altogether and eat a candy bar.
For marathons, don't eat much at all. Especially when the race starts early in the morning. Races like Boston, that start later give you time to eat a normal breakfast and have most of it cleared by the time the race starts. I found that the less I eat the better. I also found that I need to cut back on coffee or tea, because otherwise I get too wired and start out too fast. I make sure not to drink too much before the race, and not to drink at all during the first 10 miles. That avoids potty breaks, sloshing liquids and other stomach trouble.
For a 50+ mile road race on the bike, I tend to eat normally. Since most races start around 10 AM, I eat a normal breakfast of two slices of toast with peanut butter and jelly, coffee, and some banana. On the bike things are easy and if you get hungry or feel sluggish you just eat a candy bar (I prefer Clif) or pop a few gels.
For Ironman I eat early, around 4:30-5:00 AM for a 7:00 AM start. Normally I eat a bagel, some banana and a candy bar, with some coffee and juice. I try not to eat too much beforehand. I may consume a gel half an hour before the swim start, or munch on some candy bars, but overall I try to keep things light. Experience has shown that this works best, by far.
I also found that the better prepared you are, the less you need to eat or drink. I also recommend that you avoid eating big dinners the night before. Especially so-called carbo-loading dinners. These events do no such thing and are usually just an excuse to overeat and binge.