Congratulations on making leadership development a priority in your company, and on recognizing mentoring as a valuable means of providing such development. Since you indicate having previously tried a mentorship program, perhaps the place to begin is to review lessons learned from your own experience - what worked that you can build on, what didn't work and why. It might be helpful to go outside of your senior leadership and get input from many, if not all, of those who had previously participated.
Since you refer to having tried a mentorship program in the past tense, it seems that program has not been an ongoing success. Your company may, therefore, view your current attempt with more skepticism. It will be important for your leadership team to acknowledge what went wrong, accept responsibility, and communicate why you expect this time to be different.
The issue of not all senior managers being the best mentors for your core values sets off the alarms. You may want to consider sequencing your initiatives and addressing alignment of your management team first. Major change initiatives are major challenges. The odds are stacked against you in the absence of organizational readiness, and a lack of alignment regarding core values makes such readiness virtually impossible.