Ian, the basic rule is that you have to say when you are going to ride before you actually set off. The DIY organiser may accept a change to the date you put on the form, as long as you give it in advance of the ride - check with your organiser.
For the "enough distance" thing, it's often better (and easier) to plan for (eg) 205 km than for 200.5 km... and it depends which type of entry you are making:
Mandatory Routes (using your GPS device to show that you covered the exact route): usually easy to get a reliable(-ish) figure from one of the online mapping sites/planners.
Advisory Routes (using your device only to show that you did visit your specified controls): these are the ones where only the shortest on-road distance between your specified controls counts towards the 200, and this is much harder to be sure about. You'll probably need a few extra km to be sure it's enough, and on the day you'll probably ride even further (and will be able to enjoy more freedom, as well as the extra distance!)