I need to get a computer I think to monitor my average speed at least as looking at race times and terrain/altitude and distance I need to be averaging about 12-15 mph on off road to be competitive.
Completely irrelevant, I really wouldn't get hung up on the average speed.
As you've said, do a race, see how you get on. At the end of the day you can always get faster, so really it comes down to how much time you've got, you're not likely to find another 20 hours spare once you've done a race! The "how you compare to others" is a bit of a red herring really. If you destroy everyone in your first race, just move up a category.