Arguably even scarier than running a marathon itself, is the idea that you'll need to run almost that far in training - by yourself, with no crowd cheering you on. 

But how far do you actually need to run? And how can you prepare yourself for such a daunting task?

While it's important to get used to being on your feet for long periods of time, run too far and the recovery time you'll need, as well as your injury risk, will increase dramatically. The vast majority of plans recommend running no more than 20 miles in your longest run, and usually suggest doing so 3-4 weeks before race day.

For the keen beans out there, it can be tempting to try a 22 or 23 miler in the hope that this will make you more prepared than those who stopped at 20; an unknown 3 miles at the end of the race seems much less daunting than an unknown 6. Yet many argue that clocking in a few strong 16-18 mile runs is a better tactic. They claim that this is enough distance for your body to be prepped for the full marathon, and, as you are at less risk of injury and you don't need so long to recover, the rest of your training won't be as heavily impacted. 

How to choose your distance

Ultimately, the distance you need to run depends on your goal and experience.

If you're running a marathon for the first time and your main aim is to finish, focus on time rather than distance. You need to get used to being on your feet for several hours. Aim to run for 3-3.5 hours at least once, and 2-3 hours several times.

If you're a little more experienced - maybe you've run a marathon before or have a few half marathons under your belt - and your goal is to improve your speed, then get to the 20 mile mark at your goal race pace. Although there's no scientific reason as to why you should run 20 miles specifically, it will help give you confidence that you can maintain your goal speed for the full marathon.  It's also important to remember that, when shooting for a PB, the long run isn't everything. Make sure you're taking on more intense training sessions (tempo runs, intervals, hills etc) once or twice a week too.

There is no one-size-fits-all formula to marathon training. If you feel like you need to pass the 20 mile mark to be fully prepared, and you're confident you can do so without injuring yourself, then go for it. But if you don't feel like you need to, there's no reason why you can't smash race day with only 16 miles under your belt if you have trained consistently.