I have gradually come to realize that repetitive motion exercise activities such as bike riding, stairs, walking & gym exercise machines (all of the things I enjoy in life) exacerbate the pain in my TKR knee. After these exercises the TKR knee swells, gets warm, ROM is limited, and is painful. I'm not a doc but here's what I think is happening. As I flex my TKR knee, it rubs against the scar tissue that's formed (and reformed after arthro surgery) & gets irritated (I euphemistically refer to it as "angry") and the only solution is rest. Drugs and ice do little to reverse the impact of repetitive knee exercises. I refuse to give up my fitness and acquiesce to the pain but I fear continuing to aggravate the knee could cause the tendon to sever. This website discusses the issue.
https://www.practicalpainmanagement.com/treatments/interventional/successful-nonoperative-treatment-persistently-painful-knees-following
Mayo also discusses the issue here.
https://sportsmedicine.mayoclinic.org/condition/kneecap-instability-patellar-tendinitis/
Excerpt: "The pain is often worse with activities such as running, jumping, walking up and down stairs, and squatting, and it can also occur with prolonged sitting."
Given these limitations, how do I remain fit other than swimming? I found that walking upstairs is easier on the knee than downstairs, therefore, I use both handrails when walking downstairs to min the impact each step inflicts on the TKR knee. I continue cardio and strength building exercises. I build those collateral muscles that support knee functions thru targeted gym exercises and minimize the full flex of the knee that "angers it". On my TKR knee, I limit the range of the leg press to "just before" the knee pops. I use the leg press by alternatively pushing (lifting) with my toes and heels (see attached photo). I warm up extensively before my daily power walk and try to plan sedentary activities afterward. I strengthen my arms and hands to forestall falls and also to supplement the inadequacies in my TKR knee. Fortunately, I didn't TKR both knees and am using Hyaluronic acid on the non TKR knee with some success.
IMO, weight management is essential to reduce the load. In addition to focusing on weight loss, instead of unloading car grocery bags two at a time (IE), I carry the bags "one at a time" to min the extra weight on the TKR knee.

I am not suggesting any of this is easy or desirable but the surgery didn't match my expectations, I need to move past the anger/frustration, and my doctor(s) haven't solved the problem(s). I refuse to succumb to the pain, refuse to become sedentary, and am experimenting with "work-arounds" till something better comes along. Hope this helps.