I keep track of both (I use mapping software to get a close idea of distance), but I think a lot like AK- as elevation AND distance is generally what determines the time, so I always keep track of elevation in addition

time over a given distance doesn't tell the whole story in trail running like it might in road running; elevation gain/loss, trail conditions, weather, etc all impact overall time

Both, and like the OP, if I go out for a certain time at the end if I'm at 8.5 miles most likely I'll turn around and do another 1.5 miles because I have some weird problem with stopping on odd numbers and usually run even number amounts only.

I voted for distance.

I plan for distance and write down distances, but I don't sweat the small stuff.  What I mean is that I'll usually go a bit longer but round down. e.g. will run 20.28 miles, but will put it down as 20.  When I'm running while on overseas travel, I figure miles based on 9 or 10 min pace (depending on terrain) and then run for that length of time, but usually go a few minutes "extra" to make sure, plus my normal daily run pace is somewhere about 8:30.  So if I'm looking for a 5 miler I'll go for about 45 to 48 minutes.  But again I'm really thinking in terms of miles. Maybe while overseas I should think in terms of kilometers...   .