Roger, if you take alendronate before taking a drug that builds bone the effect of the anabolic drug will not be as effective.
It's L3, L4 that would be most concerning to me. Spinal fractures are the most frightening because that kind of fracture can't heal in its original configuration. The vertebral body loses height even with kyphoplasty --a procedure that attempts to restore the loss. The change in height alters the pressures and balance of your entire spine. Not to speak of the pain.
I wonder that your provider isn't suggesting a stronger osteoporosis medication.
Leg press resistance training is associated with increased vertebral pressures with prevalant injuries to the lower spine.
One trouble with osteoporosis is that your muscles can be strong enough for the weight, but the bones may not be.
You may have secured the decision to take alendronate and I wish you the best results. If you have second thoughts about taking a stronger medication, message your doctor. S/he'd be surprised and might recognize the wisdom.