Pretty much what the other two guys said.

If he is eager to have you come and is aware of your physical condition, then he probably isn't looking to fly down the path, just introduce you to hiking. That being said, a little training would be a very good idea. Walking is great, and hills are great (especially if you are going to be climbing any sort of elevation, that will sap your legs).

Important Point: If you don't know how to properly adjust a pack, (you probably don't if you have never backpacked) ask you friend about it. Have him check it when you are wearing it. This is especially important if this is going to be a longer hike and you are carrying any substantial weight. A well adjusted pack is much easier to carry, and much less tiring than something that you just throw on.

Good luck, I hope you have fun. Let us know how it goes.