"Low impact" is not the same thing as "low intensity". It is perfectly possible to do very high intensity exercise (eg. cycling) that is low impact.
While your fitness can increase quite quickly with regular exercise, it takes a lot longer for your leg muscles and tendons to adapt to the higher impact.
If you are getting pain in your knees, then that is a strong sign you should back off and look at some alternative forms of exercise (at least until your legs can adapt to the impact). Of course, if you ONLY do low impact exercise, your legs will never adapt.
Aim at moderate to high impact exercise 2-3 times per week (and keep the duration down when starting out), with low to moderate impact exercise on other days. This exposes your legs to higher impact activity, and triggers the need to adapt, while giving your body enough time to recover and for your leg muscles, tendons and bones to actually get stronger.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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