what are you walking at?
are you using a treadmill or outside?
if you are using a treadmill, increase the incline and add more speed. try not to hold on. a 15 degree incline feels great in the glutes. or try walking backwards.
if youre walking outside, try walking faster, swinging your arms, or even a light jog. can you find a more uphill terrain? if you cant sustain a run try sprint for a minute then walking again or stop and do some jumping jacks to get your heart rate up.