Yes I've suffered from that about two years ago. I was doing a lot of walking and didn't notice my shoes were wearing out. The heals had worn down on the outside and the heal support inside had collapsed somewhat, causing strain on the tendons.
I was advised by my doctor to:-
Get a new pair of shoes with good supporting soles. (Insoles etc. should not be neccessary. More important is the fit of the shoe)
Rest the foot as much as possible,
Take an anti-inflammatory (ie Ibuprofen)
Get a can of beans. Freeze them and once frozen roll the foot over them for 5 minutes three or four times a day.
Do some stretching exercises. Sit down, take hold of the toes and gently stretch them back. This stretches the tendons and accelerates the healing process.
Another exercise was :- Stand facing a wall lean forward towards it so that you are on your toes thereby stretching the foot tendons.
Do these exercises three or four times a day taking care not to overstretch the tendons.
Each exercise should last about 5 mins..
This helped me considerably but it still took about 2 months to sort out completely. ( Very frustrating)
The plantar tendons stretch from the toes back through the underside of the foot, over the heal and are attached to muscles at the back of the lower leg.
This cured the condition and I haven't suffered from it since.
I hope this is of some help to you.