I guess my first effort was the Karrimor Mountain Marathon on the Isle of Arran in 1980, followed soon after by the Capricorn Long-O - similar in navigational style to the KIMM, and often using the same areas, but being a solo event without the kit carrying. I also have had a go at the Saunders Lakeland Mountain Marathon 2 or 3 times, and the Lowe Alpine once! The Phoenix is a relative newcomer, but being in the Cheviot Hills in Northumberland, it is one of my favourites. All of these are two day events in mountainous terrain.
Locally, here on the North York Moors, the Cleveland Search and Rescue Team organise the Cleveland Survival in March each year. This is a 25 miler one day challange walk or run. There is navigation involved getting from checkpoint to checkpoint, but due to access restrictions, it usually incorporates major paths and bridleways in the circular route. In fact, if you get a leg along the Lyke Wake Walk route, you can see the path stretching ahead of you for miles and miles and miles! The checkpoints are usually easy to find too - the mountain rescue vehicles (some of which are converted trucks) and the 100ft high Raynet masts are a bit of a giveaway!
Several orienteering clubs have tried long-O on normal orienteering areas. I'm a particular fan of the British Blodslitet, a mass start event, usually on a large Lake District mapped area. Other clubs have linked adjacent areas together to get the distance in. My favourite was the event staged by WCOC back in the 1980's in Eskdale using Dalegarth West, Dalegarth East, Eel & Stony Tarn, Blea Tarn, Parkgate & Irton, and Miterdale. I recall this was a two day affair, and day one had 75 controls (in the days of needle punches - not only were there map exchanges, there were control card exchanges too!).
EBOR had a go at a couple of these events a few years back - I remember starting in Raincliffe Woods on the outskirts of Scarborough, with a link into and back out of Wykeham Forest, finishing back in Raincliffe Woods. More recently, there was an event starting in Broughton Forest above Coniston Water, taking in Caw, Stickle Pike and Torver High Common.
In the last couple of years, two new events have arisen - the Around Aldershot event run by the British Army, with a Bike-O option, and the Lincolnshire Bomber, run by the RAF O-club and the Lincoln club.
This year, I signed up for the Bomber Long-O - and what a great event it tuned out to be. Starting on South Common, near the city Youth Hostel (which we have stayed at a couple of times), the course took in a variety of rough open areas, urban parks, riverside walkways and full urban areas, before finishing (after a map exchange onto a 1:5000, sprint race symbol map) in the historic city centre of Lincoln. The old city is built at the top of a BIG hill, and after 1 1/2 hours running you certainly felt the climbs up through the old Bishop's Palace, the cobbled Steep Hill, past the cathedral, through the castle before finishing in the Lawns area.
I certainly enjoyed the concept, and middle daughter (shadowed by Mum) had a fun time on the city centre race. I do hope they run it again, and it that other clubs have a go at something similar to add to the York City Centre race in the O-calendar.