Average speeds are a bit of a misnomer in my opinion. I was obsessed with this when I started racing but you'll never achieve the average speed a race does on your own in training. One good way of judging fitness is to ride 10 mile time trials - if you're getting below 26 minutes then you're probably fit enough to race.

But there is more to it than fitness, being able to hold your position in a bunch is much more important because if you find yourself at the back you run the risk of getting dropped and it is very, very hard to get back on.