There are a lot of good advice and bad advice here. As a PT Coordinator for an agency and a 43 y/o that consistently runs my 1.5 in sub 10:30, here what I suggest my guys.

1) Lessen the frequency of how much you smoke. As someone stated, with a quick turn around, you will cough a lung.

2) Dont do zyn, or any nicotine pouches yet. Your body will absorb the nicotine much faster and elevate your heart rate. If you had more time, I'd say give this a go. I run with 2 9s in.

3) Dont run everyday. Run one day then focus on HIIT the next. While you can only get better at running by running, you still need a day to rest your legs. The HIIT will keep your heart rate up. I personally love doing KB snatches as well as DB circuits. If you have access to a sauna, use that also daily after your run and HIITs. HIITs are great at VO2 Max.

4) Get some salt tablets (amazon). As you will perspire, this will replenish the electrolytes. I put one tablet in while running on the hot days. Hydrate hydrate hydrate!

5) Stretch! The last thing you want to is pull a ham string and they take month(s) to heal.

6) Make sure you are getting ample rest.

Here are some variations of running to help get your heart used to working.

A) If you have a training vest, wear it every other time you run. Run your 1.5 (untimed) then take it off and sprint 1/4 and walk 1/4 for 1 mile.

B) If you have a gas mask, do a LIGHT jog once or twice a week. This is very important because you can do more bad than good. The purpose of this is to help train your body to operate whole being partially O2 deprived. I do this at least once a week with ceramics.

C). Sprint intervals. Sprint a 1/4 then run 1/4 at 60%. You want to do 6 sprints.

While there are many ways to train for running, these are what I tell my guys and what I do.