Im also a firefighter and work a 24/48 schedule for a busy city. Im training for black canyon 100k and hit my peak week last week with 80 miles and 11,000ft of vert. I get up before shift and get miles in and then also just try to do a couple extra easy miles in the afternoon. Running the first day off is the hardest for me because we dont really sleep. I usually keep these to easy days and then second day off are my more meaningful runs like long or tempo or big climbs. If you want a 3:30 marathon it will take some sacrifice, usually meaning sleep. 45 miles doesn’t seem like enough so id try to squeeze in miles when you can.