Hey...I actually had that in both feet this past year. I say feet because where the tendon connects to the fifth metatarsal is where it hit me the most. I wasn't able to walk right for a good two weeks after the initial inflammation.
What I did about it...
Aside from a lot of time off (which sucked), I did core strengthening religiously for half a year, and kept it up as I started running again. The stuff listed in the above post good. I also use weights or resistance on one ankle. The other non-weighted one I keep on the ground. I then do kicks with the resisted foot - forward, backward, across and away from the body. While this might be mistaken for ham or ab-/ad-ductor work by people walking by in the gym, it's actually insanely hard to keep balance (oh yeah, don't hold on to anything to stay up), and most of the work goes to your ankles, peroneal muscles, calves, arches, etc...all the stuff that needs to be worked.
I also stopped cracking my ankle. That sounds ridiculous, but I've always sort of had a knack for cracking various joints, and what I cracked down at my ankles is the peroneal tendon where it slips by the ankle joint. That can spur on discomfort and inflammation.
The big thing, though, was that I started walking/running barefoot once the pain was gone. I started from scratch. I changed from a braking and jarring heelstrike to a ball-of-foot strike.
And my two cents on orthotics, in case it comes up for you: Some PT said I *needed* orthotics, so I went along with his professional opinion, but the tendinitis in my right foot actually came up after a long walk on a beautiful WITH ORTHOTICS because the peroneal tendon is on the outside of the foot. With orthotics, there's a great deal of pressure turned over to the outside of the foot. They say that overpronation is the root cause of everything, but with a forefoot strike that's null, as now my foot doesn't turn in going from heel to toe since the heel doesn't strike.