Running is like drinking alcohol. For a start, they are both things I enjoy. There is a certain irony to this statement, as since I started marathon training I have hardly drunk any alcohol at all. But examine the evidence:
Running 5km is like a tasty pint of beer. You can do it in your lunch hour and still get on with your day. It leaves you feeling pleasantly merry and perhaps wanting a bit more.
10km is two pints of beer. It feels great and you could probably keep going a bit longer. You might even want to dance around to some banging tunes, and you’re noticeably more friendly to the people around you. You definitely feel the effects of it, you might fancy a little nap afterwards and probably have the munchies, but by the next day you are fine and raring to do it again.
A half marathon is four pints of beer. And by the fourth, you’ve moved on to a funny club that only does Stella Artois and while you were having a great time to start with you’re probably getting a little bit tired and want to go to bed but you soldier on because your coat is in the cloakroom (baggage drop) and you’ve lost your ticket and you can’t get it back until the end. However, at the end you get a second wind and convince everyone to do a tequila slammer (sprint finish). Then you are sick and pass out on the floor. You go home and spend the next day in bed and vow never to do it again. You could probably get away with this every week when you were twenty, but at forty it’s definitely a twice yearly excursion.

A marathon is eight pints of beer. Not everyone can manage this. Sensible people will not even try. Instead of going home after the dodgy club with the tequila slammers, you’re now sitting on the night tube (running round the Power Station Loop) drinking Red Stripe (Lucozade Sport), telling complete strangers that they are your best friends, listening to dodgy techno music and staggering around. You can’t remember where you live and you think you will never get home. An ambulance seems like a realistic solution to all your problems, but you have no idea where your telephone is. If you do this on a regular basis, your body will probably give up some of its vital functions and you will be banned from ever drinking (running) again, even one pint (a parkrun). Somehow, you make it home in one piece, though you have no recollection of this. You are utterly delirious. Every muscle of your body hurts. Pictorial evidence surfaces the next day and you look ridiculous, hideous and in a state of torment. You have to take time off work to recover.
You tell everyone that it was the best night (day) of your life and that you can’t wait to do it again.