Mare Won't Let Me Lead Her?

About a month ago, I got a 20 year old shire mix mare from my friend's cousin's wife's brother (I think that's how it went...). She was in pretty bad shape, so we put her in the pasture for a while.
After a while, I was going to take her out for a little bonding time. She was very friendly, but wouldn't let me put her halter on. After I finally got it on, I tried to lead her, but she refused completely. I tried every trick I knew, and finally got her to go a bit, but she throws her head in the air, pulls back when tied, and won't go any faster than a walk. (Which is a real problem when the vet tried to check her for lameness.)
Can anyone offer me some advice?

If she's been in the pasture and hasn't been ridden/worked/handled in some time, it sounds like you just need to go back to the beginning like with a young horse...lots of time together, work, and patience.

Yep, go back to the basics. Since she has a tendency to lean and pull back, I'd invest in a good quality rope halter. That will help her learn not to lean on you.

First you have to teach her to give to pressure. So pretty much like the others said, go back to basics. When you get her to take a step forward release the pressure (thats the reward) soon she will learn that if she gives into pressure it will go away. When your asking her to step forward and she doesn't, try pulling from an angle to off balance her. Any sort of step towards the pressure release, even if she stumbles into it from being pulled from the side.

Thank you very much for your help. I'll tried pulling her sideways, and it worked very well. Tomorrow, I'll have to find me a rope halter too. I'm sure she'll get it soon.