If you want to do well at field matches, you need to be good at all aspects of a target engagement (finding, ranging, engaging, follow-up actions). Some skills can be practiced alone and while matches can help with others. Understanding your gear and then being able to critically think on the clock will make a bigger difference than shooting 100 matches that aren't really the same. Most matches involve following a set CoF with known information, but executing it perfectly to do well. The field stuff often requires some thought on the clock or being thrown curveballs on your intended plan, and having some emotional durability because the scores aren't perfect.
Finding and ranging targets are something not a lot of matches do these days... but the NRL Hunter series has it involved. Practicing with your rangefinder helps, and I've noticed that those who hunt tend to have a leg up in finding targets on the clock...because they're used to looking for something that doesn't want to be found.
As for technical proficiency with a rifle, something is better than nothing. PRS/NRL/local matches/.22 precision stuff/whatever, having your rifle squared away and being able to quickly build stable positions will always be of benefit. With these, you'll need to learn to push yourself as the pace involved in a stage for one of these matches is generally too slow to get all rounds expended/maximizing scoring opportunities.
If you have the guns, USPSA or 2-gun and/or 3-gun will also have benefit with stage planning and working efficiently. Being able to quickly assess a stage/layout/information/items given and then make a simple yet effective plan is essential.
Also, if you want to do the rucking matches or be competitive at some of the CD stuff you've got to be in shape. You don't need to be a PT monster, but if you can't handle the ruck pace you're never going to make it at Mammoth or Vortex (for the rucking divisions). Even in matches where you don't have to ruck, if you're struggling with basic tasks that might be required before shooting or out of breath after a flight of stairs, it's going to negatively effect your shooting score.
Finally, shoot the matches you want to train for. I see a lot of people hold off for some imagined preparedness level, only to suffer because they didn't fully grasp what they're getting into or have over developed preconceived notions. You can hypothesize, plan, buy gear, train a certain way, but unless you really know what you're getting into it might be in the wrong direction. While team and field matches are growing, they aren't everywhere and you might have to travel to attend. The best experience is to actually do the matches. Another option here is to RO some of these matches as well.
Just my 2 cents.
