I run 9 miles every day and swim 3x per week, but am unable to run faster than an 11 min mile. What can I do to improve my running fitness?

Generally speaking running long distances at a slower pace will help build your aerobic capability. In your case though it sounds like something is holding you back...after reading your comments you are very winded running a 10 minute mile. This is abnormal for someone that runs 9 miles a day. Let's consider some possibilities:

  1. Anaemia: the act of running breaks down red blood cells. Red blood cells carry oxygen. If you are not rebuilding those red blood cells at a rate comparable to their breakdown you will end up with a low red blood cell count, and thus not be able to carry enough oxygen. This is a test any doctor can perform for you to determine if this is the case. Anaemia can be caused by many things, but if it's the running some dietary changes (and a rest period) may be all that's needed.
  2. Not getting enough oxygen: an undiagnosed or untreated asthma or allergy can cause your respiratory system to underperform. Usually asthmatics still progress, just more slowly, and are at risk of inducing an attack (should carry an inhaler).
  3. Not getting enough oxygen due to a disease. A few years back I contracted mycoplasmic pneumonia (aka "walking pneumonia") and my times dropped week after week until I finally went to a doctor. A quick x-ray confirmed that about half of my lungs weren't available to me. An extended treatment of doxycycline and a matching break from running was needed, but once I was done I felt and performed ridiculously better.
  4. Musculature: running every day may not be allowing your muscles to recover, and in doing so build stronger. Likewise you may have medical issues that inhibit your ability to build muscle. Various strategies can help this: rest days between longer runs, dietary changes that include more protein and green vegetables, weight lifting focused on whatever muscle group is holding you back, etc. As others mentioned running sprints/intervals can help this tremendously.
  5. Form: your cadence should be around or above 180 steps per minute. If you have long, lumbering steps that are significantly below this then speed requires a longer stride length. In doing so, most people land with their feet well in front of their center of gravity (usually on their heel, but I've seen otherwise). This uses a lot more energy, relies more on the strength of your muscles, and poses a variety of limits. There are many other things to do with running form, but cadence is a good indication that something is wrong.

When things are going well you should feel like increasing 10% per week (speed or distance) is a slow progression. It's usually a fight to not push harder than your body is ready for. If pushing can't get you past 11m/mi then it's time to look for significant physical causes.

Summary: physician for RBC test, and checked for other maladies such as asthma, allergies, or walking pneumonia, and consider checking in with a running coach or analyzing your form for problems.

And most of all: good luck! You have great dedication. Stick with it, work through the problems and you'll do great!

Edit: just for a data point: I'm also someone that runs every day that I can. That's usually 6 days a week, and I started long after most other folks do. I've progressed from ~12m/mi to 6:30ish in just a few years. I'm now 36, and I expect to be faster in my 40s than I am today.

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