Back in the Seventies and Eighties, we got a lot of “vague penances”. Stuff like “do someone nice for the person you offended”. At which point you’d have to explain that you have no idea who you yelled at in traffic, and they’d reassign you to do something nice to somebody random.

So it’s definitely okay to question a penance that is practically impossible, and/or hard to understand. Also, if you can’t make out what the priest actually said as your penance. Don’t be afraid to ask if you need to.
Also, if a penance is too difficult for you and you have a good reason why it would be (ie, “I can’t quit my job and take the pilgrimage to Compostela barefoot, because my kids will starve”), you can ask for a different one.
And it doesn’t have to be anything so drastic. Usually this is something like, “Father, I can’t go out at night. Can you assign me a penance I can do during daylight?” Or, “Father, I’ll faint and go into a diabetic coma if I fast that long.” Or, “Father, I can’t think of anything ecumenical to do. What would be a nice thing? At least give me some examples.”
If you can’t go back to your original priest who gave you the penance, I thiiiink you can go to another priest in Confession, tell him about the trouble, and ask for another penance. But I don’t think you can re-assign yourself one. (Pretty much would defeat the purpose.)
Hope this helps!