Step 1 was finding a race distance, so the obvious second step is adding a time to it [I was going to skip this step in the series, but it seems necessary.]
Most people think that how they train decides how they race, but it's the way you race that should detemine the way you train. At the most basic level, your goal should be to do better than you did at your last race. If you're racing the same distance - barring bad weather, injury or unusually hilly courses - you try to train better to do what you already can do and that should lead you to a faster finish. If you haven't been racing recently or are moving to a new distance, goal-setting can become much more complicated.
If you're changing distances, there are a number of ways to determine comparable times. One site that compares several of these is http://www.runningfreeonline.com/Tools/Running_Race_Calculator. The Purdy predictor is quite accurate if you're equally good at all distances. The Daniels method is often more reliable if your best distances are around the marathon. My favorite method is the Mercier-Leger nomogram, which can be found in Tim Noakes' "Lore of Running."
If you're trying to compare difficult terrain races, your best bet is to compare the times of others who have done your goal race and some race(s) you've finished. This is the method used for ultras at http://realendurance.com/ (under "compare races"). To do it manually for races not listed there, your expected finish at your goal race is your time at another race divided by another runner's time at that race times that other runner's time at the goal race. It assumes the weather will be the same and that you both went into the races equally prepared.
A goal isn't set in stone, however. As you train and get closer to your goal race, your ability to predict how you'll do should improve. Races done during training will help direct you, but there are a number of other ways to make educated guesses. Almost every coach will have some favorite predictor workout; the most famous for the marathon is Yasso's: run ten 800's (or half-miles) with equal time recoveries and your average repeat time corresponds to you finish time, e.g. 2 minutes and 50 seconds for 800 meters corresponds to 2 hours and 50 minutes for the marathon. Even cruder is to use one's average training pace and subtract one minute per mile to get one's marathon pace (which is reasonably accurate for many running under 3:30).
And then we chase the rabbit into Neverland
Finally, there's the stats addicts, of which I'm often one, who will analyze every workout. Using a log.-log. graph sheet, I'll add workouts by total time and total distance, each color-coded by how difficult they were, overlaying weeks by transparencies, to get trends. Typical average days are considered to be 80% effort, recovery runs 75%, moderate runs and easy long runs 82.5%, hard 85%, very hard 90%, time-trials 95%; this gives a series of parallel straight lines, which then allow me to extrapolate to 100% and then I find the time that corresponds with the race distance at 100%.
You won't do that... and you shouldn't! Unless and until you know how your body reacts to training and racing over a large number of years, this will just waste your time and leave you trying to decipher what looks like a bunch of random dots.
You should spend that time training instead.
23 hours ago