I'm a full value rider, in recent years often the only contender for Lantern Rouge, which didn't used to be the case and it's not me that's changed.
Riding speed between 20 - 22 kph depending on terrain, it's remained constant through two decades and several changes of bikes and kit. I do occasionally make a concerted effort at training, it makes a couple of kph's difference but I'm too lazy to keep it up. I'm reasonably swift through controls and comfortable enough on my bikes not to need much time off them. I've never managed a 10 hour 200, but have been close to it a couple of times and oddly have done a couple of 15 hour 300s.
Those who say it doesn't matter can speak for themselves, it sometimes matters to me. If you're hoping to do longer events then Speed = Sleep. I've yet to manage a 600 for this reason, I can't do more than 24 hours without a few hours in a bed and am not riding fast enough to gain that time, I've been out of time on the two attempts I've had (One by a little, one by a lot), though had no problem with the distance. It also matters if you're hoping to ride in company and everyone else is a little faster. I try not to get to the point where I'm right on the time limit, so I avoid stopping till I have a couple of hours in hand, I find the clock watching can suck all the fun out of it.