Simple pain in the Achilles tendon on contraction, particularly if you tip-toe or run, although it may settle after a couple of miles. It is often stiff on first getting out of bed.
There may be little or no thickening of the tendon, which can be tender to touch. Many runners have uncorrected pronation which makes the tendon spiral slightly under running conditions and cause unequal stresses within it.
A scan will confirm a chronic unresolved tendinitis and gait analysis with a video will show whether your pronation needs treatment.
What else could it be?
Other more severe injuries to the tendon, lower calf muscle tears or simple heel tab bruising can all produce pain within the tendon.
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 will prevent shortening of the tendon.
Your medical adviser should ensure that you have appropriate footwear, have pronation corrected, use ice, massage and possibly ultrasound 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.
Promptly treated, a few days; left untreated, it may extend to years.