Tendonitis is simply a grumpy or grumbling tendon. A tendon becomes inflamed, gets disorganised collagen fibers, gets an increase in nerve and blood vessels, becomes thick and painful. I like to say you have a grumpy tendon. Much simpler.
This happens because you exceeded its load capacity. Ie it could handle a certain amount of workload and you went past that. Now it’s a little sore, swollen and inflamed. Tendons don’t like rapid change. You need to creep up on them slowly.
Use the 10% rule. Don’t increase mileage, incline or pace by more than 10% per week. Some studies have suggested if you exceed the 10% rule your risk of injury will increase by 250-300%. Other studies have suggested that 80% of all injuries are due to training error. That might surprise some of you who thought it was your leg length, foot pronation or weak glutes. Nope, it was just plain old, boring training load.