- During training, modern athletes can use GPS watches to track distance and times. The old-school way would be to map out a route pre-mapped out route with known distances and record the start and stop times. This is of course still used in countries like Kenya. For races, they're tracked by the operators of the race. Modern marathons will put a small chip in your bib that records when you cross the start line and when you cross the finish line. There may also be checkpoints in the middle of the race that record your times at specific points.
- All world records must be run during an official race that follows a strict set of guidelines to ensure time tracking and distances are accurate. Typically international competitions like the Olympics have officials on site to judge a successful record. So you can't just call up some official at the Guinness Book of World Records and tell them you ran a marathon in 1 hour. The official rules state:
In order for a performance to be ratified as a world record by the IAAF, the marathon course on which the performance occurred must be 42.195 kilometers and measured in a defined manner using the calibrated bicycle method and meet other criteria that rule out "artificially fast times" produced on courses aided by downhill slope or tailwind. The criteria include:
The start and finish points of a course, measured along a theoretical straight line between them, shall not be further apart than 50% of the race distance.
The decrease in elevation between the start and finish shall not exceed an average of one in a thousand, i.e. 1m per km.
Because of the huge variety of tracks, you very often won't see "world records" but you'll see "course records" which sometimes can be just as important. For example, the Boston Marathon is one of the biggest races in the world, but it does no meet the criteria for IAAF. Yet holding one of the track records is an incredible feet.
- They would not wear anything to weigh them down for world record attempts. During races, there are water stations lined up at various checkpoints. If necessary, the athlete will grab and chug a cup or bottle of water while running. I don't know much about Dennis' record, but if I had to guess he just bolted straight through and dealt with the consequences while on the victory circuit. There is, of course, nothing stopping you from carrying your own water. It is quite common for people to wear sip belts or camalbacks. Though always check the rules of the race. Each race is different. For training, you could wear a sip belt or camalback as well. They will weigh you down, but it's better than being in the middle of nowhere after two-three hours of running and no water. Typically long-slow-distance runs are not about the speed and more about the distance, so the added weight is suitable. An alternate approach is to set up your own "water stations" by setting water bottles out throughout your route. The upside is there's no added weight. The downside is people will steal them sometimes.