For those of you starting out on Couch to 5K - I strongly recommend you find a suitable step (can be a door step)! and use something to hold onto, and do this excercise once or twice a day about 10 reps. at a time.
UP two, three four, DROP two three four... etc x 10
Ironically the standing on demi-tip-toe will hurt more than the drop (pictured right) but it will strengthen your achilles by lengthening your calf muscles.
Since I have been doing this my achilles 'twinge' I felt in the 'early weeks' has completely disappeared!
PB
(Model pictured: Portia, Doberman aged 4)