Tendonitis and tendinosus are two very common symptoms of overuse. They are slightly different, yet overlapping; meaning that one could very well be misdiagnosed as the other, and vice versa. In most cases, varying degrees of both would occur in the same individual, even in the same injury. For most people, these are symptoms of chronic over-use, but for a few, these can simply be the aches and pains associated with a new training regimen. In untrained individuals, some initial joint pain is to be expected, in addition to the usual delayed onset muscle soreness (DOMS). When new training elements are introduced, or when a sharp increase in activity occurs, acute tendonitis and tendinosus are common--untrained skeletal muscle and connective tissue exhaust more quickly than trained tissues, so even a daily workout will seem like over-use to a muscle that is accustomed to doing nothing.
Tendonitis is an inflammation of the tendon, resulting in stiffness and inflammation in the surrounding tissue as well. In most cases of tendonitis, the inflamed tendon is irritating the surrounding tissue by rubbing against it--think of a rope that is slightly frayed, all the little fibers sticking out every which way. In this presentation, tendonitis can be greatly alleviated by ice, anti-inflammatories, and just taking a break. Untreated or unrested, tendonitis can quickly lead to another common affliction: bursitis. Here, the inflamed tendon irritates surrounding bursaes--normally a smooth, frictionless cushion for tendons to slide across--causing pain, further stiffness, and often visible swelling of the bursa as well. Bursitis occurs most commonly in the elbows, knees, and shoulders, but can also contribute to hip pain, often in conjunction with iliotibial tract syndrome in runners.
Tendinosus, on the other hand, is a degeneration of the tendon caused by microtears in the tissue which are not allowed to heal properly. This condition also results in stiffness and pain in the joint, much like tendonitis (hence the common misdiagnosis). In chronic tendinosus, the ability of the muscle to respond to these tears becomes reduced over time. Tendinosus often presents with little inflammation directly caused by the condition, however, as tendonitis and tendinosus often occur in tandem, inflammation will most likely occur anyways. Tendinosus is slow to develop, slow to heal, and while it does have some degenerative factors, these are slow-moving as well. Regardless, it would be prudent to take all injuries seriously and train conservatively.
In untrained individuals, tendonitis and bursitis may be caused by intrinsic factors such as muscular imbalances, structural weaknesses, or a combination of those and other factors. Example: Iliotibial tract syndrome often occurs in "new" runners or "returning" runners. One possibility: Adductor, hamstring and inner quad weakness results in a chronic "tracking" and tightening in the outer quads and glutes. This, in turn, causes the IT to develop tendonitis, which inflames the trochanteric bursa (the bursa on the outer point of the hip). This results in pain during and after activity, stiffness and soreness, as well as inflammation. Other factors could include leg length differences, stride and striking differentiations, or chronic postural tendencies. In this case, strengthening the leg comprehensively through strength training would be extremely beneficial. Additionally, attention to detail in regards to the running style and form used may also help alleviate the problem.
As with most overuse injuries, the best method for rapid and permanent recovery is RICE: rest, ice, compression, and elevation. Tendinosus also responds well to eccentric loading (another reason to "feel the negative") and manual manipulation of the affected areas (massage). For the untrained individual, a continuation of training at a reduced volume will eventually help alleviate some of these symptoms.