Signs
There may be thickening of the tendon which can be tender to touch. Many runners have uncorrected pronation (fallen arches)which puts increased pressure on the  tendon when running  and cause unequal stresses within it.See Jamie our Ironman podiatrist if unsure.

Medical investigations
A scan will confirm a chronic unresolved tendinitis and gait analysis with a video will show whether your pronation needs treatment.

Self-treatment
This should include a heel raise of Sorbothane or foam, ensuring that heel tabs do not impinge upon your tendon when your toes are pointed (cutting the heel tabs off will certainly help, too). Applying ice in the form of a pack or massage and very gentle stretching to the calf will prevent shortening of the tendon.

Medical treatment
Your Podiatrist or Osteopath should ensure that you have appropriate footwear, have pronation corrected, use ice, massage and possibly ultrasound or acupuncture most judiciously. Rarely, in the chronic case, you may need the tendon surgically decompressed.

Can you run through it?
Running through it is fraught with risk, though that does not stop many attempting it.Cycling and swimming is better temporarily.

Recovery time
Promptly treated, a few days; left untreated, it may extend to years.

see your osteopath or podiatrist if unsure