Thanks for writing in with your questions. Nice work on getting your aerobic base back in the game. My thoughts on your questions:
1) I can’t see why your Z1 runs would require walking. If the top of Z2 is currently 9:30/mile and 145 you could use 130 for the top of Z1 and I would think this would still put you in the 11min/mile pace range. That’s certainly faster than a walking pace. You could keep one Z2 run/week but I’d limit the time in Z2 to around 45min. Especially once you start adding some Z3 and Z4 runs to the week. Start with some Z3 to build the muscular endurance. Something like 2x15min at 160bpm 1x/week. Build to 2x20min. After 4 weeks of this replace the Z3 with Z4. I like to start with 30-30s for the intro to Z4 work Read the article on 30-30s if you are not familiar with them.
2) I suspect you will see a nice drop in the Z3/LT pace in a few weeks. You MUST keep the aerobic base miles high or these gains will be short lived. Up the Z1 and even recovery paced run volume during the week to keep this critical aerobic support in tact.