I'm looking to run my first marathon, Pittsburgh Marathon, on May 15. I've just completed my first 50K so I have a good base to work with. I started training from scratch in September for the 50K. After the first 2 weeks of 30 miles of easy mileage, I ran a 1:34 flat trail half marathon for my first half marathon. I haven't raced a 5k in 3 years, but 3 times in the past 3 months I ran a sub 18 5k in the middle of my workout.
Is sub 3:10 an unrealistic goal for my first time out?
It depends. From training for a 50k you should have good strength. The 5k time is not that important, but sub 18 is a good preliminary indicator (you have the needed speed, now you need speed endurance). You need to get that half marathon time in the 1:25 range to able to comfortably run sub 3:10 pace. Once you do that and with all the mileage, etc., than a sub 3:10 is reasonable. You need to train to be able to hold sub 3:10 pace (Marathon pace runs of building up to 13-16 miles at goal pace will greatly help with this). Good luck