This subject is pretty taboo, with eating disorders and all. So first, Please be healthy. Also, have you talked to your coach about this?
Now, here is an article from Frank Horwill:
http://www.serpentine.org.uk/pages/advice_frank02.html
1.) As a guideline, he says a healthy, inactive female, 5'10 should weigh around 150 lbs. He goes on to say "a distance runner should weigh less."
2.) How much less? Middle distance runners (800m-10k) about 12% less. For 5'10 female, that's 132. Long distance runners (over 10 miles) about 15% less, about 128.
So, based on that (I'm no expert) you're very close. Maybe 3-5 lbs if you feel like you have a little to lose. Sounds like 10 lbs (and the 120 you weighed before) would be too light.
There are some good tips/guidelines in that article. Also, this one has some good eating rules to make sure you're eating healthfully:
http://www.serpentine.org.uk/pages/advice_frank01.html
Good luck this season. Train hard, and remember food is your fuel - you need a lot of it to train well.