What is a good time for a first-time marathoner depends on the persons gender, age, running ability and training. Size is not so much a factor and, in fact, there are distinct advantages to being small and light when it comes to running marathons (see Noakes Lore of Running, pg 180-184).
Candace, I wouldnt worry about how fast your buddies are running. A good time goal for you should be based on your performance(s) during shorter races (5K, 10K or even better, a 1/2M). Add in an extra 5-10 minutes for inexperience and even more time if its a hilly course, warm race day conditions, you werent able to train properly, you have some injury, etc.
I actually think its always a good idea to set 3 goals for yourself: 1) an ideal time goal under ideal conditions, 2) an acceptable time goal under less than ideal conditions, 3) finish the race within the allowed time limit.
Good luck with your training!
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