I'm no expert but the advice is usually to avoid long runs close together. You need to give your body time to repair itself and recover.
Another rule is to not try to catch up if you miss a run. If you can't do a long run, just carry on with your scheduled training. It will only start to become an issue if you miss several long runs. The theory is that the risk of an overtraining injury is a greater danger than the 'cost' of missing an odd run.
All this depends on your experience and fitness. Paula Radcliffe famously runs 25 miles a day in training so obviously it does her no harm, but if you're a new runner like me, the above guidelines would normally apply.
The say you should just enjoy the rest if you have to miss a few days. Your body will appreciate it as long as it doesn't go on too long.
Andy