For your training plan, does it have any explicit expectations about your current fitness level at the start of the plan? For example, must be running 20 miles per week and capable of running 10 miles etc.
If nothng explicit, have a look at the first week of your plan and work out what the total mileage for the week will be, and also what distance the longest run will be. You want to be entering the training plan knowingly you can comfortably accomplish the first week's training. If you are stretched by the first week of the trianing plan, you will struggle later in the plan due to the cumulative stress of the training.