Lower leg pain - muscle on front of shin

Sounds like it could very well be "compartment syndrome". I had the exact same symptoms for literally years. I tried everything to fix it (strength training the front of the legs by "toe'ing" up weight plates, all kinds of stretches, different runnign styles, etc) and eventually figured it out as compartment syndrome. You can look this up on the internet but it essentially is when the fascia (the covering of muscle groups) is too inflexible and cannot expand to accomodate the increased blood flow to a muscle during exercise. The lack of room to expand eventually reduces the blood flow to the muscle which in turns causes weakness, pain, numbness, etc.. The only cure for this is a procedure called a fasciotomy. I have had the done (in two separate procedures) on all 4 compartments in the leg from both the inner and outer sides. The operation involves slitting the fascia lengthwise to allow the muscle to expand. There is a test to "potentially" determine if you indeed have compartment syndrome which measuring the pressures inside the muscle compartment both before and after exercise. Some physicians recommend it but some (mine included) don't. I guess the jury is out regarding the accuracy of the test and the criteria which warrants surgery. Mine simply used my symptoms (which again were exactly like yours) to determine the cause of the problems. You may indeed have something else going on but your symptoms sound awfully familiar. At least you have some potential "source" and hopefully can avoid the almost decadde I spent looking for an answer. Best wishes! I know this sort of thing can be awfully frustrating, especially when you have the cardio capability and the leg strength to do much more.

I hear ya. One trick I used to find would occassionally work (maybe 25% of the time) was to warm up with like 30 minutes on a stair climber. After the extended warmup, I could occasionally pull off a 5K, but never anything much longer than that. My symptoms continued to get worse over the years (i.e. I could run less and less until not at all) until finally even walking at a brisk pace or holding my foot over the brake pedal when driving in stop-n-go traffic became excrutiating. One potential indicator of ECS (exertional compartment syndrome) is if you get tingling or numbness on your foot (particularly on top) or in an extreme case, a drop foot. This is caused by the pressure increasing within the compartment to the point that it impinges on one of the nerves leading to the foot. I would suspect this happens in later stages of this problem though (I was at the numbness stage but didn;t have drop-foot). Hope you are able to work it out and it isn't ECS. But rest assured, even if it is, there is a treatment which in my case was 100% effective. If my memory is correct, the effectiveness of these types of fasciotomies is in the 80+% range for anterior and lateral releases (the front and side which you are experiencing symptoms in) and in the 30+% range for the posterior and deep posterior (the calf and soleus areas, I suspect these are much lower because these are harder to diagnose correctly and can be confused with other conditions). I have had  both sets of releases for all 4 compartments and fortunately they were all correctly diagnosed and solved all the problems. In fact, they worked so well that I ran myself right into a stress fracture of the tibia late last year and am now sporting a walking cast and crutches but this too shall heal (or pass or whatever). Best wishes!