How bad is it to walk/run if you have blisters on the bottom of your foot?

Depends...: Blisters can be very painful if they still have a "raw" feeling associated because the skin has recently been peeled; however, closed blister are covered and may not hurt. I'd say it wouldn't be advisable, but honestly, it depends on your pain tolerance. Keep in mind, the more you irritate it, you don't give those sites a chance to heal.