One of the world Marathon Majors. The Boston Marathon is the world's oldest annual marathon and ranks as one of the world's most prestigious road racing events. The Boston Athletic Association manages this American classic.
The legendary Boston Marathon course follows a point-to-point route from Hopkinton to Boston. To qualify to run the Boston Marathon, entrants must have run a qualifying time at a certified marathon within the previous 18 months.

One of the World Marathon Majors. The Chicago Marathon is held yearly in Chicago in Cook County, Illinois, United States. The October 10, 2010 running was the 33rd Anniversary running of the race.
It is among the fastest growing marathon road races in the world, due in part to its largely fast and flat course which facilitates the pursuit of personal records and world record performances.

One of the World Marathon Majors. The NYC Marathon is a major annual marathon whose course runs through all five boroughs of New York City. It is one of the largest marathons in the world, with 43,659 finishers in 2009.
Today many marathons are huge media events that take over entire cities around the globe. None is as prominent as the New York City Marathon, but all city marathons are modeled on it. Modern marathoning owes its start in 1970 - and its world-class status - to New York.

The San Francisco Marathon is a loop course. The race starts and finishes on the Embarcadero (near the Ferry Building), traversing San Francisco.
The Full Marathon runs through Fisherman’s Wharf, the Marina, across the Golden Gate Bridge (and back!), through the Presidio and into Golden Gate Park. After a tour of Golden Gate Park, run down the famous Haight Street and through the Mission, Potrero and Mission Bay Districts. After a pass behind AT&T Park and a run under the Bay Bridge to the Finish Line.

Britain's longest, toughest, non-stop running race. The Grand Union Canal Race (GUCR) is run from the centre of Birmingham to the centre of London along the Grand Union Canal.
Competitors are required to complete the 145 mile distance within the time limit of 45 hours. Plenty of pubs along the way to entice the competitors off the straight and narrow (and provide sustinence to crews)

Wild and Tough - The Hardrock Hundred Mile Endurance Run is an ultramarathon of 100.5 miles in length, plus 33,992 feet of climb and 33,992 feet of descent for a total elevation change of 67,984 feet, at an average elevation of over 11,000 feet.
The race is held on a loop course on 4WD roads, dirt trails, and cross country in Southern Colorado's San Juan Range, USA crossing thirteen major passes in the 12,000' to 13,000' range.
In order to complete the event, instead of crossing a finish line, runners are required to "kiss the Hardrock", a picture of a ram's head painted on a large block of stone mining debris.

The Spartathlon aims to recreate the original 'Marathon' by tracing the footsteps of Pheidippides, an Athenian messenger sent to Sparta in 490 BC to seek help against the Persians in the Battle of Marathon.
The Greek race covers 246 km (152.85 miles) and has been held since 1983. Running between Athens and Sparta, the race starts at the foot of the Acropolis and runs out of Athens toward the coast and then along towards Corinth, where it reaches the Corinth Canal at 78.5 km. After Corinth, the race heads toward Ancient Corinth and at 159 km, reaches the top of Mount Parthenio. After which it continues south before reaching the main Sparta highway just before the 200 kilometer mark. Runners must pass through 75 checkpoints along the way and each checkpoint has a cut-off time.

A 6-day, 150 mile stage race across the Atacama desert in Chile, one of the driest, hottest and most arid places in the world.
The self-sufficiency race where competitors have to carry all their food and equipment, is staged in a place that has the most lunar-like landscape on Earth and is frequently used by NASA to test its Mars rover vehicles - what better reason to want to experience it in it's raw form. At 15 million years old and 50 times more arid than California's Death Valley - maybe Badwater isn't so tough after all :-)

135 miles in one go - Held in July each year, 90 of the world's toughest athletes run non-stop from Death Valley to Mt. Whitney, California, USA in temperatures up to 130°F (55°c)
If only because this is another race, jostling for the title of 'Toughest footrace in the world' it has to be done. Prequalification for 'consideration' is a minimum completion of 3 x 100 mile ultra-races. They don't count the MdS as it is not considered tough enough!

Arguably the greatest ultra marathon in the world where athletes have come from all over the world since it's inception in 1921. Staged in alternate directions 'up' and 'down' between Pietermaritzburg and Durban in South Africa.
Initiated by Vic Clapham as a living memorial to fallen comrades in the Great War (1914-1918), this 56 mile road race takes in the searing heat of a South African autumn as it is held at the end of May. For me this is an obvious choice, given that my wife comes from the country, but the fame, motives and history of this race make it a natural choice for any ultra-runner.

A trail race along the West Highland Way from Glasgow to Fort William, the now established route being about 95 miles (153km)
Starting at Milngavie Railway Station (7 miles north of Glasgow) at 1am the Saturday of the third weekend of June & run/jog/walk to Fort William Leisure Centre by noon the day after or 35 hours to cover 153km (95 miles) including 4499m (14760ft) of ascent.

The Western States Endurance Run is one of the oldest ultra trail events in the world and certainly one of the most challenging.
The Run begins at 5:00 am on Saturday of the last weekend in June and is conducted along the Western States Trail starting at Squaw Valley, California, and ending in Auburn, California, a total of 100 miles.
The trail ascends from the Squaw Valley floor (elevation 6,200 feet) to Emigrant Pass (elevation 8,750 feet), a climb of 2,550 vertical feet in the first 4½ miles. From the pass, following the original trails used by the gold and silver miners of the 1850’s, runners travel west, climbing another 15,540 feet and descending 22,970 feet before reaching Auburn.

The infamous John o'Groats to Land's End challenge, or Land's End to John o'Groats if you prefer. The distance by road is 874 miles (1,407 km) and the record for running the route is nine days.
I guess I've always felt that psychologically running downhill would be better but either way it would certainly be a challenge. Rory has this annual race which does the route over 15 days, running over 55 miles a day - certainly not for the faint of heart, but what better way to get up close and personal with a fantastic variety of the countryside of Great Britain in a short space of time!

The Bob Graham round of 42 Lake District fells traversed within 24 hours is probably the most demanding test of physical fitness available to British Athletes or mountaineers.
The rules for the Bob Graham 24 hour club state that the round can be attempted either clockwise or anti-clockwise. provided that the start and finish is at Moot Hall, Keswick.
First done way back in 1932 by Bob Graham, hotelier of Keswick, Cumberland, at the age of 42, the 42 Peak Round has become a testing ground for the supremely fit. Each summer around 100 of the most highly tuned ultra-distance fell runners will attempt the 27,000 ft of ascent within the allotted 24 hours. Only one in three will return to the Keswick Moot Hall before the clock runs down. Most of the rest will be back again...!