Finally got a bike but my knees hate me—any gentle stretches that helped you?

I just bought a secondhand hybrid bike to get some low-impact exercise without pounding my joints. I’m in my late 50s and haven’t been very active lately, so I’m excited but also a bit sore after a few rides. My knees especially feel stiff and achy, which I didn’t expect since I thought cycling was supposed to be easy on them.

I’ve tried adjusting the seat height and taking it slow, but the discomfort lingers. I don’t want to give up because I know getting moving is important, but I’m worried about making things worse. Has anyone else dealt with knee pain when starting out cycling? Did certain stretches or warm-up routines help you? Or maybe modifications to your bike setup?

Also, how long did it take before the aches started feeling better? I’d love to hear your experiences or any gentle moves to ease into this without aggravating my knees.