The strange thing is that it didn't hurt at all after the Mt. Diablo 50K. I took Sunday completely off other than some walking and ran 3 easy treadmill miles Monday evening. On Tuesday I did 7 miles that felt really good. Wednesday I had to cut my run to 4 miles due to some GI distress (bad lunch food choice coming back to get me). I don't remember the foot hurting then, but I do remember that when I went to run on Thursday it was already hurting and I wasn't surprised. I pushed through the run, but it hurt more near the end and I finished with a modified gate which is never a good side, but further confirmed my tendinitis diagnosis. I took the next 3 days off, but I felt it all weekend.
1) New work shoes
I bought some new shoes for work last week. They are slip-on Clarks and seem to have a decent insole and OK support. However, they are a little different from my previous shoes which were Borns and had a bit of a wider fit. I prefer to wear slip-ons as I take my shoes off when at my desk. I am going to try a different pair of older shoes for a little while.
2) Yoga DVD
I recently bought a DVD called Yoga Conditioning for Athletes which is actually quite good. While I think it is unlikely to be a primary cause, it probably didn't help as any of the one-legged stands on my left foot became increasingly difficult. I have stopped the conditioning for now, but will restart once I feel confident in my foot/ankle.
3) New running shoes
These would seem like a VERY likely candidate prima facie. However, while they are a new model they are an update of my older model that I have been using for a few years and they fit very similarly. The new Brooks Adrenaline GTS 7 really didn't change much over the 6 except in the cushioning technology. That said, I must admit that after running in my 6's today (which I was planning to reserve only for trail running), I definitely feel more isolated from the ground in the new 7's and didn't feel as good on uneven paths. I will stick to my older shoes for the time being, but if it is the shoes I have a bigger problem long-term :-(.
4) Too much too soon (too asphalt-y)
While I didn't feel any problems immediately after my race, I did jump back into running with only one rest/recovery day. It was a tough 50K and perhaps I didn't respect the impact on my body and take more rest than I normally would after a "mere 31 miles". I also have been running on the road due to life and time constraints which is never preferable. After taking the weekend off I feel like I can run so long as I take care of my foot/ankle after and don't push too much into the pain zone.
If the modifications above don't continue to show improvement then I know that a longer break period is definitely called for whether I like it or not. Tahoe is a long ways off and it would be MUCH better to take time off now than closer to the race.