@Emilyharmsen Welcome to the community! I started trail running about four years ago sort of on a whim. That turned into a 12k trail race, then a 25k mountain race followed by a trail marathon (through Oregon forests in ceaseless rain) and then a 50 miler. My advice for running a marathon? Do it! Pick one you think you'll enjoy—I choose races based on scenery and location—but for others it may be the finisher's medal, community or personality. If it's something you look forward to it motivates you to get out and train!
My second bit of advice is don't worry about pace/time. Lots of egos sprint away from the start of races only to be caught and passed down the road. Sure, train hard and set realistic goals but don't get wrapped up in finishing in x minutes/hours for your first race. Trail marathons in particular, with varying elevation and terrain, make pace meaningless. I've run 50 miles in under 11 hours, but also in just under 18 hours with long sections of knee deep snow, waist high river crossings and 60 degree temperature swings.
Where there's pain there may be problems with form. I discovered (by accident) that yoga—specifically cat/cows—provided form awareness I needed to overcome plantar facia pain that plagued me for two years. It also improved my economy and speed. The trick for me was learning to tuck my hips back and tighten my core. That altered my hip position, which altered my posture, which altered my foot strike so I land on my midfoot/toes instead of my heels. A coach or a good PT person may be able to help you there, too.
Starting out I suggest finding a local group to run with. Perhaps some running shops near you organize weekend or morning/evening runs. I think what's important is to try out multiple groups. Each will have a personality and you'll find people in each to offer advice, challenge, motivation and company. Something to be weary of is listening to one person or group. Get multiple opinions and find what works best for you. If someone insists "Brand Z shoes are the greatest" maybe smile and move on! After a while you'll find you mesh with individuals and maybe the group runs hold less value. I started with a club run of 20-40, found a group of 5-10 regulars that turned into two close friends. They've moved on and now most of my runs are solo adventures with my dog. I prefer the solitude and quiet (and liberty to go when, where and how far) it affords me.
Fanny pack or vest... Tried both, several different brands of vest. I love the Salomon vests. They fit snug without binding or chafing and I forget I'm wearing them (I own three, each different capacities). They hold things in place without any bounce and I find I can carry all my dog and I need for 6-10 hours in the mountains.
For hydration, I personally like the Nuun Endurance products. They have mild flavors that don't taste like chemicals to me. I almost always add a shot of cocktail flavoring syrup (blackberry) to my bottles and the Nuun berry mixes well. I won't drink if I can't take the taste and that combo works for me (plus the dog likes it, too). I supplement hydration with salt caps, one/hour. On longer runs I fill a bladder with clear water; the Salomon vests have insulating sheaths for the bladders that help them stay cool on long days.
Where to run? I'm lucky. I have hundreds of miles of trails accessed from a trailhead 50' from my door. My suggestion is to find someplace nearby and remove the "takes too long to get there" excuse, at least for "routine" runs. A familiar loop is great for workouts & intervals. Apps like TrailForks and AllTrails can guide you to popular trails near you, and Strava has features for discovering segments and routes where others run, and your local running shop/REI can probably help out there, too.
Hope that helps! Good luck on your marathon!
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