The upper part of the leg contains the quadriceps muscles. I don't know if your soreness is there or on your actual thigh. Concerning what to do depends a lot on your body, your schedule of running, how much rest you get, your shoes, and so on. Here is a general comment. Give yourself a couple days of rest. Then cut your distance back to 10 miles. If you can run at that distance then run 10 miles for a couple of weeks. Then advance your distance in small increments. This is known as the 10% rule. Some people can do 11 or 12% while others can only do 7 or 8%. The number 10 is just a typical value and not a magic number. The key thing is to advance your distance in small increments such that your body can adjust to it without causing soreness or pain.
Other questions concern your weekly schedule. How many time per week do you run and how many miles each run? Total weekly miles? A good suggestion is to run one long run each week (e.g. the 10+ miles), one medium run of about 75% of the long run, and the rest of your runs short, easy, recovery runs (e.g. 5 miles). Your long run and your medium run put a lot of stress on your body, and you need the recovery run to help you recover. Another important principle is to run heavy-stress/light-stress. That is, you run your long or medium run, and the next day should be a short recovery run or some cross training or even a rest day if you feel tired. Your body needs at least 48 hours to recover from your heavy-stress runs, and recovery runs or cross training or rest days help give that 48 hours.
Running actually destroys body cells, and it is during the rest or low-stress days that your body reacts to the stress and rebuilds the body cells. So, not enough rest means not enough recovery, and that is a path to injury.
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