Ah, the familiar old adage: "winter miles are summer smiles". An insight into the standard held belief in the endurance community that more = better. Wether you're a runner, cyclist, triathlete or Ultra-marathoner, you're likely familiar with the guilty feelings associated with one too many beers or missed training sessions. The knee jerk reaction of many athletes is to train harder/faster/longer next time out. You need to make up for lost ground and by going harder/further, you'll catch up to where you need to be.
Unfortunately, training attitudes such as this are the reason physiotherapists are doing so well. A huge proportion of running injuries occur when athletes re-start training following a break, as athletes we tend to overestimate our capacity for adjusting to a new stimulus. Likewise, it's all well and good following a training plan, there are plenty of generic programs out there. But what happens when you miss a workout? Or two? Or you don't have the time one week and have to cut your long run short and skip your speed work? This is when changes in load (load = a quantifiable calculation of work done during a training session), can become silent wreckers of best laid plans.
In basic terms, the body is protected from overuse injuries by the load it is currently used to. Small changes in load aren't cause for concern, however big increases or decreases are important. Changes in load cause adaptation responses in skeletal muscle, ligaments and tendons, however it's the discrepancy in recovery time between these structures that is important. As most people are aware of, muscles recover from a hard workout in around 48hrs, and It's pretty easy to know where you are in that cycle. Just poke a runners legs the day after a half marathon PB and you'll get a clear response! However, whilst muscle tissue can be back to 100% capacity in just a couple of days, ligaments and tendons take much longer. This is due to their collagenous nature - the repair and maturation of the collagen fibres that make up ligaments and tendons can take up to 2 weeks. Hitting another hard training session when you're still in this recovery phase is a dangerous game, repeated exposure to loads you aren't ready for will result in injury.
Similarly, lay-offs from training can have the same effect if not managed properly. Time away from the repetitive impact of running has a weakening effect on ligaments and tendons: The collagen within them weakens and conforms to the demands placed upon it (i.e not very much!). Sure enough, if you're off sick for 2 weeks and jump straight back into your training program, your tendons/ligaments have lost their ability to transfer load in a safe manner. They also won't recover fully in time for your next workout, starting the injury cycle off again.
If you're in this situation right now, don't bother doing a double session on your first week back. The only thing that will be better for it is your ego, and you could be facing an uphill struggle as a result. Instead, think back to how far you ran in your last "perfect" week, and figure out your average pace for the week's distance. Take between 50-65% of that distance, and run at 65-85% of the average speed. If you usually run once a week, err on the side of caution and use the lower boundaries for distance and speed. If you run twice a week or more, do what you feel comfortable with. If you've had more than 4 weeks off, do the very easiest level. Repeat this for 2-3 training weeks, before returning to your "perfect week" distance. If you regularly do speed work (Intervals/Sprints), consider that load is directly proportional to Distance and Speed. The faster your run, the greater forces being taken on by the joints/ligaments/tendons. Add these sessions in only when you're back up to the distance and average speeds you were running previously.
Remember, its better to go into an event slightly undercooked than be bed bound by tendonopathy!