So, I'll start with the direct answer to the question folks ask first: "Chickpea, but do you run that far when you're having your period?"
Yes. Yes I do.
First of all, can you honestly believe that people who menstruate (non-binary folks and trans men, I see you) are weak 25% of the month? Yeah, me neither. While there's some interesting science suggesting that we might want to adjust our approach at certain points in our cycles, nothing really indicates a need for sweeping changes or complete rest. Of course there are days that we're running a higher temperature, feel sluggish or may have some physical discomfort. If it's worth it to you to work through to solutions so you can get out there and run, then do it! If not, get some extra sleep. But make your choices on how you feel personally, not some shaky science-adjacent advice.
In fact, I'd suggest that our periods are practically superpowers. We have a biological process that repeats about once a month, that gives us a status check into our overall health. If your training is changing your period, especially if your period stops, it's time to reexamine your training. Athletes who menstruate, especially in sports like running in which certain body types are idealized, are vulnerable to RED-S, relative energy deficiency in sport. Folks of all genders are getting bad or dangerous advice from coaches who want their athletes to make drastic changes in body composition. Although missing periods is only one symptom, it's the most obvious symptom of RED-S. It's our superpower that our body can check in and tell us on a monthly basis whether we're ok. And if we're not ok, late or missing periods can get us to start a conversation with our medical providers before we get the more serious, long-term symptoms of RED-S, like metabolic and bone injuries.
If you're on a hormonal treatment or a medication that manages or disrupts the endocrine system, that may affect the regularity of your period. Make sure your providers understand your athletic training and can help you monitor yourself for RED-S. Not every training plateau or bout of fatigue is a problem, but it's better to have objective eyes on your progress to avoid potential problems. Don't stop taking prescribed meds! Periods are great monitoring tools, but they're not worth sacrificing other aspects of your mental or emotional health.
While dysmenorrhea can be a primary medical condition, it's also a potential indicator that something else is wrong too. Again, our superpower is telling us to get help! Any improvement in underlying health is an athletic improvement, so go get answers!
And what of the more routine stresses and discomforts during our menstrual cycles? Personally, I find that running decreases my cramp symptoms--if I can make time for a run when the cramps hit, I usually feel better for the rest of the day. Other symptoms, like a higher body temperature, I tweak my hydration plans to manage. And if you're the lucky menstruator who experiences GI disruption and diarrhea, carefully timing your runs around mealtimes (or public bathrooms) can make it manageable. There's some trial-and-error at play in solving these problems, so listen to your body and be patient while you figure it out. Eventually, you'll be able to run comfortably for longer even on the tough days.
So then let's circle back to the big question about long runs. How do I manage my flow when I'm measuring a run in hours instead of minutes? I started using menstrual cups for environmental reasons and they turned out to solve my running problems too!!! The best one for me has been the Intimina Lily Compact because of both the fit and the portability--it takes the same space in my pack as a couple of tampons but works far better, far longer. There's a learning curve with a menstrual cup, but after a couple of periods I never looked back to disposable options. And even if you need to try a couple different menstrual cups before you find the right fit for you, you'll still save money in the long term.