Without wishing to state the obvious with boots you do get what you pay for in terms of materials used, build quality and longevity. In the real world £69.99 boots will buy you nothing but problems and disappointment.
In my opinion the Keen Explore Mid Waterproof Walking Boots are nothing more than a very lightweight training shoe. Whilst many Keen users state their footwear to be comfortable Keen definitely do not have a good reputation for build quality or remaining waterproof for any length of time.
From your last reply you seem to have already made your mind up but if you do buy these I would suggest that you will be buying new replacements again inside of 12 months. If you are happy with that then so be it as it's your money and your choice after all and I will try very hard not say I told you so.
If your username is an indication of your location I'm surprised that you haven't already taken advantage of a visit to Whalley Warm & Dry to seek their knowledge, skill and expertise to help you with your knee problem and your quest for comfortable boots? They are widely regarded as one of the best bootfitters in the country. Granted they are not cheap but as I said before with boots you really do get what you pay for.
https://www.whalleyoutdoor.co.uk/https://www.whalleyoutdoor.co.uk/boot-fitting-service/My recommendation would be that you increase your budget and arrange to visit Whalley Warm & Dry. In the meantime have a look at the following boots
2-3 Season BootsAltberg Fremington Men's 1412g RRP £184.99 (standard last with 5 width fittings)
https://www.altberg.co.uk/boots/5-width-fitting-boots/fremington-men
Best online price £166.49 @ Taunton Leisure
Altberg Malham Men's 1344g RRP £189.99(A-Forme last with 1 medium width fitting)
https://www.altberg.co.uk/boots/aforme-boots13/malhamBest online price £170.99 @ Taunton LeisureAltberg's standard last is available in 5 width fittings from Extra Narrow to Extra Wide. If the model of a boot made on the standard last doesn't quite fit or suit your particular foot shape, width and volume it's a fairly safe be that a boot made on the A-Forme or G-Fit lasts will. All Altberg boots can be resoled if and when the need arises.
The budget boots mentioned in this earlier post ~£100 may also be worth checking out as possible alternatives
http://www.walkingforum.co.uk/index.php?topic=40880.msg591554#msg591554