How to train for 1st full marathon

find a good training plan - hal higdon, Galloway are 2 that I recommend (although personnaly I lean towards Galloway because I run/walk most things)

agreed with @TavistockToad - you need to run more than twice a week, three at a minimum, most marathon plans will have you 4 days a week - suggest switching at least one of your ellipticals for a run

more time in the lead-up to your marathon the better - it will give you time to repeat a week if needed (i.e. if you have a 16 week plan, and 20 weeks until your race - you could do an addition month of base training, or you could sprinkle re-do weeks in between)

stick to the plan - if the plan says run 10 miles, do it, don't exceed it (I know that its tempting to do) - plans are structured a certain way and if you increase too much too quickly

take advantage of your long slow runs - these should be 2-3min slower than anticipated race pace (so if you are aiming for a 10min mile - then LSD should be 12-13min miles)