Since you are in an aggressive training program for a marathon please read on 'runner's knee' No matter what other problems you have this could at least help with your knee pain Your idea of walking during periods of pain is good as well as swimming.
Runners Knee (also called Patellofemoral Pain Syndrome, Patellofemoral Stress Syndrome or Anterior Knee Pain Syndrome) is one of the most common causes of knee pain in all age groups, including teens and young adults.
It can be chronic or intermittent. It is common in runners, hence the term "runners knee", but also occurs in inactive people.
Runners Knee is a condition characterized by pain behind or around the kneecap. Poor kneecap tracking is believed to be the main cause this condition. The kneecap (patella) slides over a groove on the thighbone (femur) as your knee bends and straightens. If, for example, the front thigh muscles (quadriceps) are weak or imbalanced, the resulting muscle imbalance can pull the kneecap to the left or right of the groove, causing pressure, friction, and irritation to the cartilage on the undersurface of the kneecap when the knee is in motion.
Overuse/overload of the quadriceps - especially running, going up and down stairs - can cause this condition to flare up, as can poor exercise techniques, e.g. a poorly fitting bicycle, improper footwear etc.
Causes of Knee Maltracking
There may be multiple factors involved. Muscle imbalances/weakness or inflexibility in the muscles that support the knee, and mechanical errors can cause poor knee tracking. Muscle imbalances in the quads are especially common. In females, the increased inward slant of the thigh towards the knee is believed to the reason they are at higher risk of developing Runners Knee.
Mechanical errors include misaligned joints in the foot or ankle; a kneecap that is located too high in the joint; flat feet / over pronation. Pronation is the normal inward roll of the foot as the arch collapses after heel contacts ground during walking or running. Over pronation causes excessive internal rotation of the lower leg and knee.
Muscle Weaknesses/Imbalances/Tightness Include:
Weak Quadriceps: Quadriceps (front thigh muscles) strengthening exercises are considered to the most important exercise, in most cases, for correcting poor knee tracking. The quadriceps controls the movement of the kneecap. (They are attached to the kneecap and then to the top of the shine bone by tendons).
Imbalanced Quadriceps: Sometimes the quads (there are four divisions) are imbalanced. The inner quad pulls the kneecap inwards and the outer quad pulls the kneecap outwards. If the inner quad is weak, the stronger outer quad tends to pull the kneecap off center. In this case, exercises to strengthen the muscles of the inner quadriceps are particularly helpful
Weak Hamstrings: Another imbalance can occur when the muscles in the front of the thigh are significantly stronger than the muscles in the back of the thigh (the hamstrings). If your hamstrings are weak, your quads have to work harder. Tight hamstrings cause increased pressure between the patella and femur.
Tight Iliotibial Band: The iliotibial band (a fibrous band of tissue on outer thigh that extends from the hip to below the knee) also affects knee stability. If too tight, this muscle/tendon of the outer thigh can pull the knee to one side. A tight iliotibial band can also cause Iliotibial Band Syndrome (see Knee Injuries).
Weak Hip Abductors: The hip abductors (muscles on the outer thigh involved in moving leg to side) also help support the knee. Strengthening these muscles may also improve runners knee.
Runners Knee Symptoms
Pain, typically diffuse pain, in front, around or beneath the kneecap. More pain and/or feeling of joint instability after climbing stairs, jumping rope, running, or after a period of sitting.
There is extra pressure between the kneecap and thighbone when the knee is bent at a right angle as when sitting. The kneecap is pressed towards the femur. If there is already irritation of the cartilage on the underside of the kneecap, discomfort or pain results. Even sleeping in a curled up position can cause pain when the condition has flared up. Contracting the quadriceps, as when going up or down stairs, also causes increased compression of the knee joint.
Sometimes after activities that have activated the quadriceps, it feels the knee is being pulled to one side. Sometimes a clicking, cracking or crunching sound is heard when the knee is bent or straightened. This is the kneecap slipping back into the groove.
An X-ray or MRI of the knee can show if there is damage to the cartilage or if the patella is displaced or tilted. A tilted patella may be correctable with exercise if the tilt is caused by a muscle imbalance.
Runners Knee Treatment
Rest: (not total rest) Temporarily avoid activities that cause extra stress on the knees such as squatting/kneeling or high impact activities like running until the pain subsides. Swimming or low-impact activities such as working out on an elliptical trainer are fine. Avoid squatting/kneeling as a bent knee causes extra pressure between the patella and femur. Avoid leg presses where you support your weight with a bent knee. Straight leg lifts are safer. As you get stronger, partial squats are ok
Icing: Applying Ice to the knee, especially after exercise may reduce pain and swelling. Don't ice for over 20 minutes at a time to prevent frostbite. Elevating the knee above the level of the heart while icing helps in reducing the inflammation.
NSAIDs: non-steroidal anti-inflammatory drugs such as Aspirin or Advil, as recommended by your doctor.
Exercise: Exercises, particularly exercises to strengthen and stretch the quadriceps (front thigh muscles) and hamstrings (muscles of the back of thigh). The exercises emphasized in the majority of cases are those that strengthen the quadriceps particularly the inner division of the quadriceps. This usually is very effective. Spending a few minutes, a couple of times a day on these muscles and gradually working up to 20 minutes per day are sometimes all that is needed. Be patient. It can take several weeks to notice an improvement. See Knee Exercises.
Knee Taping: Taping is used to realign the kneecap and hold the kneecap in place. Although knee taping has not been scientifically proven to help stabilize the knee joint, it has been shown to significantly reduce pain. The relief is usually immediate. There are different techniques that a physical therapist (physiotherapist) can show you. The tape can be irritating the skin.
Knee Brace: a Patellar Stabilizing Brace helps keep the kneecap in the middle of the patellofemoral groove. This may be helpful when the muscles than support the knee are still weak. A knee brace can take some stress off the knee and help relieve pain. Wearing a brace does not replace the strengthening exercises that correct the root of the problem. Braces help some people more than others. (Wearing them during sports has not been shown to reduce knee injuries) They are expensive, and some people find them hot and bulky. Patellar stabilizing braces must be fitted properly to be effective. Not all knee braces are created equally. Ask a doctor or physical therapist whether or not knee braces are appropriate for your situation.
Proper Foot Wear: e.g. Shoes with an arch support to control over pronation, shoes with adequate cushioning in sole to help absorb shock. Orthotics may be required for those with severe over pronation. See Knee Pain, Overpronation, and Footwear page. High heels throw your body forward and increase the pressure underneath your kneecap. Limit the time spent wearing high heels.
Surgery: Surgery for Runners Knee should be a last resort, after an exercise program to correct muscle imbalances has been given a fair trial. It may be necessary if there are significant structural abnormalities.
Arthroscopy and Lateral Retinacular Release: If the knee-tracking problem is caused by excessive lateral pull (kneecap pulls toward outer side of knee), cutting the tight lateral ligaments to reduce the amount of pull can rectify the problem.
Patellofemoral Pain Syndrome / Runners Knee may lead to Chondromalacia Patellae.
Runners Knee is usually easy to treat. Doing the appropriate exercises and avoiding exercises and activities known to cause undue stress to the knees are usually enough.