Avg target distance is around 450yds over 20-22 stages of the matches I've been to in UT,WY,MO,ID,CO and CA. I know
@hic28 calculated the avg distance for his match and it was around that figure.
I've seen targets as close as 140yds and as far as 1000yds and change, the MDs have been doing a good job of sizing the targets appropriately for the distance, a small target will be in the .2-.3 mil range and generally a big one on the .5 - .7 mil range. Most matches are a mix of animal shaped targets and your standard geometric targets.
When it comes to mistakes, equipment management and ranging/land marking are two big ones. Using a tripod on a prone possible position or going prone on very un-even terrain... those things will get you and correcting will do you in time.
Some matches do a better job than others at keeping stages blind so you'll see a lot of equipment mistakes on matches that do. Also, got to come in with a lot of confidence in your dope. Missing high on a stage and not knowing if its equipment/data related or just a bad range will carry over for stages if you aren't sure about your dope. Can't stress that enough.
As far as Cal a lot of people (if not most) run some form of 6.5mm. 6.5CM is popular and I've seen a lot of .25 CM too. Its power factor based so that's where your tie braker will come from. A lot of suppressors a lot of breaks a lot of different barrel lengths.
Its great field shooting but its gear intensive too. No one is out there competing with a sling or coyote sticks and having a good time. You can do well with a .5 moa gun all day. A good tripod, a Double o triple pull bipod and a good set of LRF binos will make the whole experience that much more enjoyable.
You can shoot one for less than half the regular fee with whatever rig you have in whatever cartridge. That'll give you a idea if it's for you. or you can attend as a spectator for free and hover all over the place with a squad.