Zanax and lexpro don't work well together. Zanax is also not a "rescue" when an attack comes on, more like a maintenance thing daily. If your panic attacks are that bad you should have an emergency script for klonopin for an insant chill. CBT works well over long term.
DO NOT DOUBLE Up on Zanax thinking it will work, you will black out or bring on addiction leading to alprazolam rams(paradox reaction). Black out is scary enough without going into a blind rage, both which can be deadly operating any machinery including bike. Use with caution and know it will stop working for you at some point even at high doses and coming off Zanax can do serious harm. Take a look at reviews on drugs.com to see the pattern.
Seriously, you have to learn to deal with PA's in an organic manner, crutches only make the problem more apparent.