weak quads - what to do?

[bfit]

I have always had weak quads (and I don't know why), but now that I'm in my 50s and recover more slowly than before it's bothering me more. If I do a lot of running or step my quads get sore and tired, and then if I keep exercising through that I get sore knees (probably I assume because my quads are too tired and stiff to support my knees and keep everything aligned the way it should be). If I'm sick or otherwise take a break from exercise when I start up again nothing else gets sore except my quads. I do stretch so I don't think they are unusually tight. Unfortunately, it seems like a lot of the exercises that are supposed to strengthen the quads like squats and lunges also make them tired and sore and then I'm into the old vicious cycle with sore quads and sore knees and not being able to do any quad exercises. Does anyone else have this problem? Any ideas on how to strengthen my quads without hurting myself or my knees?