Cardio with overused hip flexors

So I've been doing the couch to five k, and apparently I OVERdid it, because now my left hip flexor is killing me, and has been for the past two weeks. (I stupidly tried to push through it up through this weekend, when I finally realized I was actively hurting myself.)

Here's the thing -- I don't want to give up doing cardio while I let my hip flexor rest (I do strength training twice a week), but what is some good cardio that doesn't strain it too much? As an added complication, I have piriformis muscle/sciatic nerve issues, so bike riding will just send me into a whole other tailspin of pain.

Any ideas? Thank you!