For my shorter runs (4-6 miles), I rely on my Garmin Forerunner to tell me the distance. I set the alarm to sound when I'm half way through and then I turn around.

For my longer runs I usually go for time, but sometimes (like today), I choose a route that has a "known" distance (such as in a mtn bike book or whatever) and run it. The one I did today was shorter than I thought it would be and took less time. I wanted to run for 3 hours and it only ended up being 2 hrs and 20 minutes. That's a whole lot less on the trail.

I don't compare to "road" times so I have no clue about pace. My pace depends on the trail, how much climbing, how hot it is, etc.

The great things about trail running are (1) it's not about your pace (2) you can walk the nasty uphills (sometimes you HAVE to) and (3) you reap a lot of benefit from the movement of your entire body, not just your feet hitting the pavement. Since I've started trail running my hips and ankles have gotten noticeably stronger. Not that road running is bad...I just can't do it anymore because it's just too painful (mostly my back). Trail running is a whole 'nother animal, as I've been learning this year. I can't imagine going back (I probably couldn't, actually).

Have fun and take it slow. It takes a while to retrain yourself to run on the dirt.