There are a vast number of national events and here are a few!
The Three Peaks challenge comes in variety of formats - and can be tackled individually or as part of an organised event click here for more info!
The most famous marathons and half marathons are the London Marathon and the Great North Run.
Entries for these are generally by ballot - but guaranteed places may be available.
If you are thinking of taking on one of these challenges you may care to have a look at this website which offers free advice on training for marathons!