Does anyone know what I'm doing wrong?

I have used the treadmill in the past and had to spent 2 months with my knee in a compression bandage. I only walk, it's only for 20 minutes and it's not brisk enough to even warm me up!!

I have trainers, I put a tubigrip on my knee and wrapped up warm as the treadmill is in the garage where it's a bit cold admittedly.

Would using the incline facility help at all? After yesterday's efforts my calf muscles ached and still do!