I use a Garmin 305 unit. They make a Gamin 205,305, and 405.
The 405 is brand new, but the 305 has been out for a while. The 205 is more what you are looking for. The 205 doesn't come with the heart rate monitor strap like the 305 does. Im not sure what the new 405 does yet. Im sure its just a bit smaller than the other 2. The 205 and 305 are the same unit, different color and a few more options than the 205. They work great and I use mine every weekend for trail running. Once you start the unit you can have it retrace you back the same way you came, and also mark spots on the trails for water or anything you can think of. I wont list all the things they do, but its a bunch. The cost for the 205 is around 125-175 dollars. You can get them at your favorite sorting goods store, or I would suggest nextag.com. They have many vendors that have them. Good luck, Im sure you will like them. They are about 2 times bigger than a regular wrist watch.