Building up steadilyJust as nobody should go straight into an ultra, the same applies to drinking. Typical advice to a new runner would be start with walk/runs and the drinking equivalent is a shandy at lunchtime. It helps get drinking into your routine and can build up alcohol tolerance in a fun, sociable setting with no pressure. You can move onto beer (maybe two). This is equivalent to your first parkrun.
As you gain confidence and tolerance, you might want to move up to the "few beers after work" level which is more like a 10k. It attracts the more serious drinker and helps to build stamina. Now not all evenings are the same - "leaving drinks after work" can be quite aggressively paced and can be considered up there with a fast half marathon.
The work Christmas party can be for many the highlight of their drinking year. However just like the London marathon, it can be the downfall of many unprepared people who haven't done enough training and really shouldn't be there. It is easy to get dragged along by the crowd at such an event and set off at a pace you aren't used which ends in tears. It usually features fancy dress.
Specificity of trainingFor a successful ultra, it is important to match your training to the course. If you are training for UTMB, jogging along a canal will not do you much good. Find out about the event you are training for. Ideally find out the drink of choice for the stag/hen and recce the effects of it - you might be quite comfortable getting up to 10 pints of bitter in a session but you will be quickly undone by a few glasses of Aperol and Prosecco if you aren't used to it.
Useful training sessionsThe "quick beer before the train home" is quite good training. Getting 3 pints in before the 18:07 from Fenchurch Street helps gain many of the effects of a good interval session
If you are getting bored of regular beer try a night out on Belgian lagers. They are much stronger and will make it easier to drink normal strength lager afterwards
This strategy can work very well training when you don't have time for a full session or are training for multi-day events. It involves a big night out on Saturday followed by a few pints at lunch on the Sunday. This can be quite difficult to justify with significant others but the Sunday effort is particularly good training for stag dos.
Food / soft drinks
When you start to get beyond "a few beers after work", a nutrition strategy is essential. Some people sneak in carb based drinks such as coke but this is open to ridicule from fellow drinkers. Ordering food can help to maintain performance levels and to slow the absorption of alcohol.