I haven’t done 200mi, but I’ve done 48hr and 72hr races.
What I’ve found to be really valuable training is doing double run days (during several years of the 100 runs in 100 days), and doing a slightly modified version of the Goggins 4x4x48 challenge (my segments were a bit more than 4mi each). Getting comfortable with getting up after a rest and running again on legs that absolutely do not want to work, and especially with doing so while sleep deprived.
Expect to walk a lot. TRAIN to walk a lot. You won’t get better trained at walking by running, so make some of your training several-hour hikes, preferably with similar climbing and descending to the course. Adding a bit of extra weight to a pack while you’re hiking can help.
Most people will use poles for 200mi races. Train with the poles, on all sorts of terrain - your shoulders will thank you later. Figure out if you need to wear gloves to keep your hands from getting munched.
Expect to get way colder than you think you will at any given temperature, especially at night. Have warm, dry layers in all drop bags (and I mean right up to puffy jackets even for summer races). Fresh socks or a change of shoes (to give hotspots a rest) will help - for later drop bags have shoes that are a half to one and a half sizes larger than your usual size to accommodate swelling. Baby wipes and a toothbrush and toothpaste in a few drop bags, too - it’s hard to overstate how much more human you feel if you can wipe that crust of salt off your face and get the coat of fuzz off your teeth.
For crew, you want people who will both parent and bully you. You will protest doing things that you should be doing (eating/drinking/grabbing gear) like a toddler having a tantrum - they must not capitulate to your petulance. They must also not be put off by helping wrestle someone who smells like the dumpster behind a fast food restaurant on a hot summer day out of their clothing and/or shoes and back into fresh duds. Knowledge of foot care (lancing and dressing blisters) is very helpful, too, and you should have foot care stuff like duct tape or leukotape in every drop bag, plus lube - ALL THE LUBE.
Extra batteries for headlamps are great, but you can’t use the headlamp you’re taking the batteries out of to help you see how the replacement batteries go in. Either carry a second light source, or just one of the little single-use glow sticks; they weigh heck all and will throw just enough light to let you work.
Make a plan for your crew access aid stations, but discuss with your crew that the plan may need to change. You’re not going to know (unless you’ve done this a bunch of times) if you’ll be able to stomach the thought of the burrito you thought you’d want at mile 136, so have other options - LOTS of options - for food. Sweet, salty, different textures (especially soft things like melted ice cream in case your teeth/jaw are sore), and definitely hot food for the nights - a thermos full of broth doesn’t have many calories, but it will warm you up when you’re shivering and stiffening up. Hot chocolate will do the same with more calories. Be mindful of too much fiber, and avoid spicy foods unless you’re really sure about your belly. Know that bowel and bladder control become god-tier skills after 25 or 30hrs of movement, so try not to test your limits with anything you’re taking in (also another reason to have baby wipes readily available).
When you nap, try to do so with your legs elevated. If you’ve removed your shoes while napping with your feet up, tie them loosely when you get up because your feet will start to swell again once gravity takes over. You’ll immediately get chilled upon waking; try to get moving again as fast as possible to warm yourself up.
Lol that’s probably enough of a novel for now
