It is frustrating. But I have found when I have had (occasional) back issues, that riding a bicycle is almost pain-free when walking is agony. So your metric of walking at your usual pace may not be the best measure in this case.

I am not sure why riding a bike should present less pain, but I am not the only one to feel this. patentcad's story is much like this, too, plus others I have talked to in person.