Quite frankly, aiming for a half IM is a good goal, but ask yourself if it is a realistic at this point in your training? A solid Half IM training plan should include 16 weeks of base training followed by 3 build phases and a taper.

Take your time and train properly. Trying to do too much in too little will ALWAYS lead to injury or overtraining. Then you risk further set backs. It's a no brainer....proper training is a progression and this can NOT be rushed. Finish this season doing your sprints, and gradually increase your training distances thru the off season. I also think it may be in your interest to complete at LEAST 3 OLy's before doing a half. If you are injury prone, yuo might want to consider the guidance of a certified coach.

Regarding your injury, I am not sure what to make of it as there is not a lot of information about where the pain is, etc. However, having pain around that is not localized well and is worse after sitting for periods of time with the knee flexed is classic for Patellofemoral pain syndrome (also known by various names such as chondromalacia, patellar compression syndrome, quadriceps insufficiency syndrome). Generally, this syndrome falls under the broad class of syndromes called "overuse syndromes" and as you know the treatment of choice for any overuse syndrome is rest and avoidance of any activity that causes the pain for several days to weeks. This does not mean avoidance of exercise, just those that cause the pain. Applying ice to area involved 15-20 minutes 3-4 times a day and religiously after exercise also helps because whenever you have pain you also have inflammation and ice is one of the best and also cheapest anti-inflammatory there is with the fewest side effects. I use a bag of green peas, $1.50 and reusable. Rest is the first part. The next step is strengthening the quadriceps muscles. If you are still having significant pain you want to do this with the knee in extension. Do straight leg raises every hour while awake adding weight as you feel you need. Either lying on your back or sitting extend your knee and raise your foot. Feel your vastus medialus to be sure it is contracted. Hold this for 6-8 seconds and repeat 4-5 times. STRETCH. Stretch your quads in AM, before and after exercise and before bed. Hold stretch for at least 20-30 seconds to the point you feel pulling not pain. Use Non-steroidal anti-inflammatories for at least 10 days.