Well, the feeling of having to go is your penis filling her colon, this is normal. The best way to get around alot of the feeling is to make sure she is relaxed. If you both are putting too much thought into the act itself, well you will never relax enough. Dont make this the main course for her, like maybe get her off another way a few times IE give her oral or finger her g-spot maybe even use a nice dildo, this is to help relax all the muscles in the vagina as well as the anal sphincter region. Make sure to use plenty of lube, this is key. You probally should use a condom, and remember never go ass to vagina, she could get a really serious infection. The other key is to maintain the distraction from anal intercourse throughout the entire act, like even when your moving in and out she could use a small vibe or her fingers on her clit to keep her mind focused on her feelings, this should help it not hurt as much and will keep her anus from tensing up. The most common reaction when a woman is having and orgasm is to push out on the vagina and anus and then pull in kinda like a milking affect. Let her do the work, the man could lay down and she could get on top that way she can take as much of the penis or as little of the penis as she wants. There are alot of good sites out there that can help with anal sex. I know when my wife plans for me to use her ass she wants me to start small with like one finger and then two, this helps get her muscles opened up for her to take in me. What you see in porn is so far off, the act where they go ass to mouth, noway, what they dont show is the man pulling out and the crew handing him a antibacterial wipe and him getting cleaned up, and then stroking it til he is ready to cum and then she puts it in her mouth. So if your basing anything about anal on what you see on the screen dont. Try a website like this, I have found alot of info at this site www.holisticwisdom.com