It's always a challenge to match up schedules for shakedown events.
At our initial meeting we explained that we expect participants to attend the majority of the shakedown events. If they are unable to do so, we understand, but they won't be going on this year's trip. Knowing that up front helps set the expectations.
It may be too late, but you should at least consider letting people know that if they don't participate in the shakedown events they are at risk of being dropped.
Also, it's imperative that the adults are in shape. They are usually the ones who underestimate the challenge and arrive out of shape and slow down the group. That's just not fair to the boys.
We try to have more smaller events on different days and at different times so we get the maximum participation. We have very few events that everyone can attend at the same time.
We also have Philmont mini-meetings at the normal weekly meeting. That's a time that most of our group has blocked off for scout related events anyway. It's not much fitness stuff, but that's when we did the trek selection, and we go over gear, cooking methods, water purification, etc.
What is the age and experience of your crew and when is your trek?