In the beginning, I just aimed for 5k in 30 mins. Over the years, I've been pushing that time down, chipping away at it, completing my first 24:xx 5k in the summer. Have lost a bit of fitness now and have been eating too much shite, so nowhere near that at the moment! I would say it depends what you want? Would you prefer a longer distance or a faster time over a short distance?
I had a very pleasant hour on the trails this afternoon - muddy, hilly, slow...