5k: 5k events are very popular and achievable by almost everyone, one way or another. There are many charity events, corporate challenges and even regular events like parkrun at this distance. Many of these are happy for you to walk, if you prefer or need to, and most people can walk it in about an hour. It is also the target of Couch to 5K, so finding a 5k event is likely to be the first thing you want to do when you compete the programme, if not before.
10k: If you are doing 5k runs regularly, you may be looking for something a bit more challenging in terms of a race distance. 10k races offer that: still run at a hard pace by the eager runners at the front, but achievable by the beginner and without needing weeks of training like the longer distances. This is the longest distance run on the track in official World Athletics races, where it's normally referred to as a 10000 meter race.
10 mile: An unusual distance for a race because of its closeness to the half marathon, many people enjoy this distance as a training run for half or full marathons or just to see how far they can push the threshold. Often offered with a 10k alternative, you may find yourself wishing you'd gone for the "easy option" when the 10k'ers peel off as the marshal directs you on farther!
Half Marathon: A favourite distance for many runners, there are plenty of events to choose from around the country, so you can probably find one near you. This distance still offers runners a chance to push themselves hard if they like, but most races will give you plenty of time to complete it at a more leisurely pace. Do check for cut off times, especially as some races aimed at club runners may not have the resources or budget (e.g. for road closures) to keep the race going until the last person is in.
Marathon: Talk to a UK non-runner about marathons and they may think there is only one in the UK and it's in London. In fact, you could probably enter a marathon somewhere in the UK every week. The limited space and increasing popularity of entering for a ballot place at the London Marathon means it's highly unlikely you'll get in that way. If you are determined to do the London Marathon, the easiest way to guarantee entry is to apply for a charity place; but be prepared to commit to raising several thousand pounds for your chosen charity. If charity fundraising isn't for you, you could join a running club that has a draw for a few places allocated to it, but you may find that you have to be a member for more than a year to be eligible to enter the draw.
Ultra-Marathon: Formerly a niche player in the mass-participation running scene, the ultra is becoming increasingly popular and new ones are frequently appearing. As running a marathon is starting to lose its wow factor, the ultra marathon is becoming popular with those looking for something a little more challenging. Anything over the marathon distance is technically an ultra marathon and there are quite a few 50k races around. However, most of the serious ultra marathon races start around 50 miles, although the distance is often irrelevant as almost all of them are on the trails and often involve many hundreds, if not thousands, of meters of ascent.
Obstacle: If running is a bit boring for you, maybe you would like a few obstructions in the way? Obstacle Course Racing (OCR) is another growth area in the sport of running and even has its own governing body. Race lengths vary, with some even reaching marathon distance.
Cross Country (XC): Although it may bring back bad memories of your school days, many people enjoy cross country events during the winter months. In the UK, official races are often open only to club teams, but there are some public races available if you prefer to run as an individual entrant. Distances vary, although there are some standards at the more elite end, which have been the subject of much debate as the female races have typically been capped at a shorter distance than the male race at the same event.