Could you maybe offer to walk together just once or twice a week and reserve the other days for yourself for more intense walking or other exercise? I'm not sure what else to suggest. It's a tricky situation. Do you think she realizes she's holding you back a bit? It may be frustrating and feeling tricky to her too. Good luck!
And welcome to the board! We're glad you've found us. Tell us a little more about yourself, if you don't mind :)