Oo, walking is super safe and honestly the best alternative, walang pressure sa cervix at kaya ninyong gawin every day kahit low cervix kayo, just listen to your body kung parang heavy ang feeling. Since your OB already advised against yoga, I'd stick with that and maybe ask her din kung okay na swimming or light stationary cycling, kasi those are gentle options na maraming moms swear by sa 2nd trimester, pero since you have low cervix better to check with her first para sa peace of mind mo.