What cardio should i be doing if i have posterior tibial tendon dysfunction?

Many choices: Bike, swimming, weights are all possible. If there is no pain, just weakness or flatfoot can even do elliptical or treadmill. Any exercise that causes pain should be discontinued. May benefit from orthotics.

PTTD: You can do anything as long as you have no pain or symptoms otherwise seek an opinion of a physical rehabilitation specialist to assist you.

Anything but ...: With true pttd, your arch is collapsing repetitively with every step and you need support. Without it, in the long run, you'll develop other problems. Have it looked at to see if there are other causes for the pttd.