I personally find the difference between 50 miles and 100 is huge, and I need to do longer rides to ramp up for a century. So if I were you I would use one day per week to do progressively longer training rides, up to 80 miles long if possible. That will also give you a chance to compare the two bikes over longer distances to see if the added comfort of the CX bike is worth the added energy it takes to ride it.

If you don't already know the potential reststops along the trail, I would research them in advance, to figure out the best opportunities for food, drink and bathroom stops. The less you have to carry on your bike, the easier it will be.

Also I understand why you're planning this in July but if the trail has long unshaded sections you'll need to figure out if they will be a problem for you. There's a nice MUT here that is 45 miles long and parts of it are lovely but other sections have no shade at all, and they just bake in the sun during the summer and are quite unbearable in the middle of the day.

Good luck!!