While I won't claim to be any kind of expert, I have eight different 5Ks now under my belt, plus a few races at other distances. No marathons, but one half.

In comments, JoyMama asked me something I wish I had known at my first 5K. She's about to do her first one. It got me thinking about how far I've come from those days. I had to go back to my running log to figure out which race we were even talking about.

I'm not sure that any of this actually comes under the heading of things I wish I had known. Maybe one or two.

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I didn't know...

...how often I would go back and look at my old running logs
There's one good thing about being a major geek.  I started keeping running logs right off when I started running.  I refer to them far more often than I ever thought I would.  Thanks to this nerdy habit, I know that my first race was actually 2.75 miles.  I ran it at an 11:05 pace.  My first 5K, I ran at an 11:59 pace, finishing in 37:15.  Two months later, I chopped my time down considerably to 29:35, a 9:31 pace.  

I never expected how useful it can be to go back and see what your times were on a particular running route at a particular phase in your training.

...the "race day" routine
In a funny way, this was the most anxiety producing during my first races.  Heck, it can produce anxiety even now.  Find the right line.  Tell them your name.  Pick up your packet.  Some races will break this process down into multiple steps.  Bag. T-shirt.  Timing chip.  Number. Use four pins (they'll have them there).  Don't pin the bottom part of the number in case they need to rip it off of you when you come through the chute.  Of course, if you have a fancy pants running number holder thingy like Brother Metronome, you don't have to worry about safety pins.

I always worry about what to wear on a particular race day.  What's the weather going to be like?  Am I going to need Body Glide?  Pants or shorts?  Do I wear my beloved running tights?

...that a good strategy is to dress warmly for the start and then dump some of your clothes at the last minute
Of course, this is complicated in the winter by the fact that you usually end up standing around for at least a  half-hour.  I've learned to throw on an extra pair of sweatpants over my tights or shorts, and then dump them.  Larger races will have a formal bag drop.  Smaller races, you can usually just stash them somewhere likely.  I stashed my sweats under my car, once.

...about the differences between small races and larger ones
I remember someone saying something mildly disparaging about my first race.  It was put on by some sorority on campus as a fundraiser.  Looking back on it, it's pretty clear that they didn't know what they were doing.  They didn't have a lot of the amenities that I've come to expect. I didn't care.  I didn't know what I was doing, either.

Big races will have timing chips, although they're going to disposable RFID tags on your number instead of chips you attach to your shoe and have to return.  In a bigger race, your "chip time" counts from when you cross the start line to when you cross the finish.  Larger races have more swag in their bags.  This is generally of dubious value.

Medium sized races won't necessarily have a pad at the start line.  You only get a "gun time," since there's only a pad at the finish.  I don't mind this setup much.  Being computerized helps them keep track of runners and post times more quickly after the race is over.  I can mentally deduct however long it takes me to get from where I started to where the actual start line is.

The smallest races won't post your time at all on the web.  This is important because you need to look up as you get close to the finish line, find the clock, and then actually remember what it said.  This is a task that I find surprisingly hard to manage at the end of a race.

...that slow people start at the back
Embarassingly, I didn't really clue into this until my second or third race.  I mean, I was never right up front or anything.  I just didn't catch on that the fast people go in the front and me, I go in the back.  I usually try to eyeball people and sort myself into being just in front of the walkers.  Guessing wrong about your pace relative to everyone else means that you're either getting in someone's way or someone is getting in your way.  So far, I've only been at one race that had signs up for particular paces.  I wish more of them did that.

...to think about my splits
In some sense, this really doesn't matter for your first 5K.  Just run as fast as you can, right?  Still, it's nice to know how you're doing along the way.  Some races will have someone calling out times as you reach mile markers.  If your race publishes their route ahead of time, you can figure it out yourself and pace yourself with your watch.  Cool running has a nice pace calculator that will also do splits, and tell you how much time it takes to get to particular mile markers at a given pace. 

...nobody could tell that I didn't know what the hell I was doing
I thought sure that everyone could tell that I had never run a race before, but of course that wasn't true.  Even people who look nervous can be veterans.  Hell, I still get nervous before every single race.    

...that there's some sort of cruel conspiracy to always end races uphill
What's up with that?  It's just not right.