These races are a perfect warm up option for those attempting a marathon. They allow to see how you cope with your chosen marathon pace in the context of a race and also to practice and fine tune your preparations such as getting your gel strategy correct without going quite as far as a marathon.
Many training plans suggest completing several twenty mile runs in the training phase and so using a race as one of these long runs can give an added focus and add that extra bit of excitement and competition to your plan! The round number of 20 miles is used in the U.K and U.S but in Europe the more common long race choice that is not a marathon is the 30km. Many European cities have urban trail races that are very popular and can be very tough in terms of length, elevation and terrain underfoot.
The 20 mile warm up race is the perfect time to make sure all your race day kit is just how you want it for the marathon. Just don't get too carried away if the 20 mile race is not your actual goal race and don't do one so close to your marathon that you don't get chance to taper properly. The races show below are all over the country so you should be able to find a suitable one nearby.