Don't worry. In the course of sixteen or eighteen weeks, there will be things which happen. You need to respond sensibly. If you're happy to get on a treadmill and feel better about it, go ahead, but going short on one workout won't blow the plan.
Today is Saturday. That means, if you have both days of the weekend off, you could run long tomorrow. If you were considering today's planned 15 as a long run, it should have been done at an easy pace, so there is nothing physiologically to stop you from running long tomorrow. Of course, do not have a hard workout planned for Monday if you go with that option.
Remember, plans are no more than reasonable suggestions as to what should work. Unless you're an elite athlete with a carefully designed series of races designed to peak at precisely the right moment, remember the reason for the various components of the plan and be prepared to respond to personal circumstances (illness, work challenges, etc.) or the weather. If you become obsessive compulsive about following a training plan, you run the risk of unnecesary injury and besides, your friends will think that you are becoming boring.
Have fun whatever you choose to do.
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