Simple pain in the Achilles tendon on contraction, particularly if you run on your toes, although it may settle after a couple of miles. It is often stiff on first getting out of bed.
There may or may not be 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.
Addition of hill work or speed work may cause achilles tendon problems. Improper training techniques ie: too much too soon is also a common cause. Following the 10% rule for long runs and weekly training runs.
What Else It Could Be
Other more severe injuries to the tendon, lower calf muscle tears or calf muscle tightness can all produce pain within the tendon.
Heel raise of Sorbothane or foam. Applying ice in the form of a pack or massage.
Appropriate footwear, have pronation corrected, and use ice, massage and ultrasound. Physiotherapy may help to reduce symptoms and correct the cause.
Can You Run Through It?
Do not run through it.
Promptly treated, a few days; left untreated, it may extend to years.