Most ultra marathoners (in fact nearly all, other than the top elites) use a run/walk strategy for ultras. Many runners use it for shorter races too. I’ve used it for all my ultras, and for several of my marathons – coincidently (or not), one of the marathons I used it in has been my fastest to date.
There are many common run/walk strategies – the one I use for ultras is 25/5 – twenty five minutes running, followed by five minutes walking. Some people use 20/5 (I used this in my last marathon), 9/1 etc.
There are a couple of important points about run/walking:
Do it in training – it is very tough on the calves until you get used to it, and trying it for the first time on the day of a marathon or ultra is probably a very bad idea. Many people find it tough to run/walk during training, especially if they train alone (“look at that asshole, he told me he runs ultras, but here he is a mile from home, walking”), I know I do – it’s much easier if you do it with a group, and entrust one of the group to time the run/walk times, and dictate the pace.
Do it from the start of the race – waiting until you are too tired to keep running is too late to start a walk/run strategy – that’s called “being too tired to run” not “strategy”.
The walking bit of the run/walk is meant to be brisk – it’s not a chance to take selfies, admire the view, or check Facebook – you should decide what pace you need to walk at (around 14 min/miles is good) and stick to it. The walk times should be used for taking in food and drink, applying Vaseline to sore areas, and general maintenance, but all while still moving.
When you are exhausted, the temptation can be strong to extend the walk break, or add in an extra one – try to resist this, and focus on getting to the next (legitimate) walk break.
While the fixed time ratios above work for most situations, you need to take terrain into account in deciding your strategy – on hilly ultras, and trail ultras, you need to adjust the ratio so that you are not running uphill and walking downhill! I have done a couple of short course ultras where I walked the hills and ran the rest of the course, and this can work with many courses.
There is a really good run/walk pace calculator here that can be used to determine what pace you need to do to finish in a certain time.
A week-planned Run/Walk strategy can extend the distance you can run considerably, can prevent or postpone exhaustion, muscle cramps, and stomach issues, and can reduce the recovery time after a race significantly. If you haven’t tried it, you should.
More advice on marathons and ultras here.