Is exercise and sport or rest better after knee surgery?
A common question asked by patients is what sport/ activity can I continue after my knee replacement surgery? The short answer is that I would encourage you to stay fit and active as the benefits to your overall health (lower risk of cardiovascular disease, stroke, diabetes, dementia etc) far outweigh the theoretical risk of premature failure of your knee replacement. This topic was debated at the American Academy of Orthopaedic Surgeons Meeting in 2017 with an expert panel consisting of Richard Berger, Keith Berend, Jess Lonner and Tad Gerlinger.
It was universally agreed that patients can safely partake in low-impact sports with no restrictions. Examples of these activities include walking, hiking, cycling, swimming, yoga, fishing, bowls, doubles tennis, and snow skiing (blue runs).
The panel was split about advising patients about high-impact sports such as running, triathlon, soccer, hockey and singles tennis. Richard Berger and Keith Berend advocated that patients could partake in these high impact activities without risking premature failure of their knee implants. Jess Lonner and Tad Gerlinger advised their patients to refrain from these activities if possible, however, they did acknowledge that passionate runners will continue to run regardless of the advice of their doctor. No implant-related failures from excessive activity had been seen by any of the panel, except from one patient who required a change of their polyethylene bearing in a unicompartmental knee at the 10-year mark (Unbelievably, this patient had competed in the Hawaiian Ironman Triathlon after their knee replacement surgery.)

Running after knee surgery.
No data in the literature supports the idea that excess high-impact sport will cause premature failure of your implant. However results from the Australian Joint Registry do show that younger patients have a higher rate of revision surgery (failure), which most surgeons would presume would be partly activity-related.
In my own experience, patients who want to continue to run after knee replacement surgery in Perth are in the minority. I recommend you continue doing all the activities and sports that you are passionate about with the caveat that if you plan on competing in the Hawaiian Ironman you probably will have a slightly higher chance of implant-related failure over a 10 – 15 year timeframe.