Toying with the possibility of a 50K?

Several months ago one of my favorite running buddies told me that she was interested in ultrarunning. My husband is an ultrarunner, so I told her she should talk to him about it, and she spent hours looking at the various websites and blogs he recommended re: long distance training and races. She got hooked. I encouraged her to go for it but thought to myself, "No way in hell am I running anything longer than a half marathon." (I actually did run a full marathon once, but it was a really long time ago and I didn't train properly. Painful.)

But a seed was planted. Then it was cultivated by several simple changes in my running life and personal life, and by my natural penchant for challenging/torturing myself:

• I started running with a group. Almost everyone in the group is naturally faster than I am. Although I am still a lackadaisical runner when left to my own devices, and although I often feel like I am on the verge of puking during our group workouts, I can tell that my pace has improved significantly over just a few months. 

• Running with other people has stripped away a lot of my natural social inhibitions and anxiety (with said people). In other social and professional situations, I find myself trying to be whoever I "need" to be to fit in. That's a long-held habit that I am working to break; it's not conducive to developing solid friendships because people never really get to know me (in fact, I barely know who *I* am in those instances). But when I run, I'm myself. I'm so focused on keeping up and simply getting through the workout that I don't have time to think about how I'm coming across. And sometimes, not thinking is a great thing. I've started to form solid, sincere, and supportive relationships with my running partners. (I'm not saying everything is always hunky dory. I struggle with friendships. But these glimmers of being able to "be real" with people have been encouraging.)

• I started running outside on a more regular basis. Running on a treadmill is fine for me: I don't usually mind the monotony, and it gives me a chance to listen to music and kind of zone out. But running outside is physically way more challenging. It's also less predictable and therefore way more interesting.

Making these changes leads me to wonder what else I am capable of doing when it comes to my running. And now I, too, find myself thinking about longer distances. I'm toying with the idea of training for a 50K with my afore-mentioned running friend. True, she's speedier than I am, but we share the same drive to challenge ourselves, and we work well as a team. It would be a way for me to push myself as a runner and as a way to strengthen bonds with someone I think I'd really like to know better. 

I think a 50K is doable, especially if I aimed for a race in spring 2013. I could train over the fall when the weather is cooler and have something to look forward to other than school-related stuff. And if we tacked on a couple of extra days, it might be a good mini-getaway. If I used my frequent flier miles, I could do it on the cheap.

Question: Does anyone have a suggestion for a good spring "starter" 50K in the U.S., preferably one that is at least partially on trails?