Due to back issues, when I run my back jarrs and jams up leaving me painful

I am looking into getting a recumbant exercise bike which is one of those bikes that has the back supports when you're sitting down. They're meant to be very good for ppl with back pain. Im saving now

What is considered a reasonable length walk? Is 30min the norm? I live in a slightly hilly area - its not gigantic hills but my street is a steady incline to get to my house haha