It might be a fun addition to your walking program but I don't think it would be a replacement. Piecing quilts involves mostly short lengths therefore it's lots of start, stop, start, stop.
The only time I did enough treadling to qualify as exercise was when I sewed for several hours every day for three weeks straight. I was in a rush to finish some rag quilts for last minute Christmas gifts. My calves would be sore by the end of the day but would be fine the next morning.