SimOz1, you have a quite similar injury to me (ACL tear), (mine was ACL, MCL and PCL but without a meniscal tear). So, from what the GP, surgeon and physios have all told me - you can do anything in a straight line! The ACL's weakness in your knee will become apparent when you stop and pivot on the affected leg. I don't know anything about the meniscus tho. I can't hike either for the risk of slipping

- I've already had my surgery (1 month post op now) and have to wait for another 11mths to do any real hiking (well I'm gonna bug my surgeon about that when I next see him). BTW, you have a LONG recovery ahead of you. If you have private insurance YAY! because I waited 10 mths for my op thru medicare. So total time out from hiking = 2 years.
So straight line things you can do are e.g. bike riding and stuff that doesn't involve the knee like kayaking which you're already doing. It is good to have strong muscles around your knee to support the joint, so bike riding would be good for that. I didn't do anything for my year awaiting surgery (sat around and sulked because I couldn't play basketball), I have lost a lot of muscle and gained a lot of fat lol - but oh well.
RonK - I also get serious knee pain on descents and perhaps you and I have different problems but…I always thought the issue was with my degeneration in my knees (I have been wearing orthotics since I was 15 due to wear n tear on the cartilage causing bone on bone grinding). But apparently with 20 years of orthotics wearing these days my knees are very nice n healthy inside - surgeon told me my knee looked perfect inside apart from torn ACL and some meniscal bruising - both from the injury. My sports physio diagnosed my knee pain on descents as patello-femoral pain which is a treatable condition with strengthening exercises of the vastus medialis oblique (vmo) (i.e. inner quad muscle). I had all the tell-tale signs of the condition which I will work on fixing in conjunction with my ACL rehab. (The signs are basically - weak, underdeveloped vmo, a knee cap that doesn't sit flush on the knee when leg straightened - because the outer quad is pulling the knee cap away from the centre), tight ITB, hammies and calves and also flat footedness). I am yet to find out whether these exercises will help. Apart from that, descending REALLY slowly and using walking poles prevents flair up.