Definitely focus on stamina first, especially for your first marathon. Perhaps your biggest training goal, aside from actually being able to cover the race distance, is finding your pace — what you are comfortable running at on the big day. Lock that pace in so you don’t get overly excited and go out much faster and then bonk. My wife nailed her first marathon by just sticking to 9:00/mile or 5:35/km through the second half of her training. Alright, with that clear, the advice toofasteddie and I are giving is primarily about your overall health through the training program. Running one speed for that many weeks will be mentally taxing and can lead to overuse injury in some muscles/joints and weakness in others (the two are often related). So doing faster intervals now and then is for maintaining physical strength. It gets you to open up your stride, using wider range of muscles, and also keeps you a little more flexible. And, it’s just a change to the routine which is important for your morale. Someday, you might want to try going faster, and then those intervals workouts, which you will have good experience with by then, become your tool for pushing your pace goal. Make sense? We still recommend speedwork even if you aren’t trying to run faster at this point. So if you’re pace now is 5:00 (which is great!) then just drop it to, say, 4:45 for some 400s. See how that feels. Then the next time try a little faster if it feels good. Feels good is the key at this point in your training and your race goal.