I think it's great that you're pushing yourself out of your comfort zone, that's the only way you're going to improve. It's totally normal that you'd be sore, simply because you're muscles are used to running 3 miles as opposed to your recent four. When you push your body hard during a workout, your muscle fibers tear. While traumatic only on a small level, these micro-tears are still painful and cause you to feel sore after a workout. After a hard workout, your body works to rebuild itself. The micro-tears in your muscle fibers need to be repaired, and your body not only mends these tears but creates additional muscle fibers as an extra layer of protection. As your body creates these new fibers, you gain muscle and build strength.
So that means you need to give your body enough time to recover, but that certainly doesn't mean that you can't run. I suggest that you actually do go out for a run today as opposed to taking the day off, you'll be less sore after you finish. Just keep in mind that you have to run slow, taking it too hard can cause serious damage to your already damaged muscle fibers. That said, you're the only person who knows your body, so if you think you should take the day off then that's what you should do, honestly, you could either way. You won't stat loosing muscle after a day of rest.
One way you can give yourself a little relief from your soreness is icing. Experts say indirect ice -- an ice pack wrapped in a thin towel -- is best for immediate relief. Don't use heat, it will feel good while it's on, but it's not going to lessen the damage or make it go away anytime soon. Cryotherapy, or cold therapy, when used in the properly can diminish pain, metabolism, and muscle spasms, thus lowering the inflammatory response and improving recovery. I suggest icing your leg at least twice a day for about 20 minutes. Fill a ziploc bag with ice, place a dry towel on your leg and just let it sit there for awhile, it'll speed up your recovery and you'll be able to get back to pain-free running sooner.
Another way to dull the pain is by taking acetaminophen (Tylenol) or a nonsteroidal anti-inflammatory drug (NSAID) like ibuprofen (Advil, Motrin), naproxen (Aleve), or aspirin to help relieve the discomfort. Just be cautious about using NSAIDs regularly. Hope that helps! Good luck!
· 6 years ago