Cycling is a good, non-weight-bearing exercise to
keep the supporting structures around the joint strong. You need to
contact the doctor and talk about pain management options. A knee brace, compression wraps, ice packs, pain patches, physical therapy, Synvisc injections, etc. At your age it wouldn't surprise me if the doctor is reluctant to consider a partial knee replacement (I had my first hip done at 37), but it's best to exhaust every other avenue of pain management first.