Given an infinite amount of time almost anyone could complete even 1000km of riding. The challenge in official events like London Edinburgh London is to do so within the time constraints set.
As the length of ride increases riding faster can become harder and harder. However, with the right training it is possible to increase your average ride speed.
Good ways to do this are:
Targeted speed workouts to improve key fitness markers, in particular your Lactate Threshold and VO2max (although these are tricky to measure certain training regimens seems to have a positive effect on these two turning points). Whilst you do not need to ride at this intensity for your long events, making fitness gains at these levels will increase your overall speed at a lower intensity.
Long endurance rides and/or increased volume at an easy intensity. The more riding you can do at an easy intensity (below your aerobic threshold) the more efficient you become (the faster you get at riding for the same heart rate). You can also keep going for longer with less muscle fatigue.
Given that the clock does not stop it's a good idea to keep track of elapsed time rather than riding time.
Having a list pinned to your cross bar for cut off and the maximum elapsed time you have to get there will also help keep you on track.
The time you spend off the bike cannot be regained or made back. This doesn't mean that you should not stop; it would be dangerous not to, but keeping those stops to a minimum is far more efficient than assuming you can stop for longer and ride a bit faster to make that time back.
With practice and careful planning you can minimise the time you spend off the bike. Initially this might just be about getting used to spending more hours in the saddle without a break by learning, amongst other things to eat and drink while you ride. The following tips will help ensure your stops are efficient and your riding time is at its maximum: