Cycling is very good for strengthening your knees and for general fitness. Use an exercise bike or traffic-free cycle routes if you prefer not to go out on busy roads.
If you get a lot of knee pain you may have to take it very gently to start off with. Stop if your pain gets worse after cycling.
Cycling is good for your heart and health – everyday cycling, where it leaves you breathing heavily but not out of breath, is an effective and enjoyable form of aerobic exercise. Cycling can improve your mood and have positive effects on how you feel – reducing levels of depression and stress, and raising self-esteem.
Sustrans has some useful information on cycling in Cornwall https://www.sustrans.org.uk/ncn/map/walking-and-cycling-inspiration/best-regional-routes/cornish-coastal-routes and has a very useful tool to show you maps of on-road and off road cycling trials http://www.sustrans.org.uk/ncn/map
Other cycle routes in Cornwall can be found here http://www.cycle-route.com/routes/Cornwall-Routes-68.html