|Road marathoning helps with all forms of running.|
One of the staples of ultra training are back-to-back long runs. However, I don't favor the long, slow run except for beginners who need to gradually increase mileage. For runners who can comfortably run a 20-miler and have run several races of marathon distance or beyond, keeping some elements of speed in the long runs makes sense. That's even true for back-to-back long sessions.
Renato Canova trains many of the top Kenyans and his athletes run long runs close to marathon effort. If they want to run a marathon at 4:45/mile there's little benefit in training the body to run much slower than that. For them a comfortable long, slow run might be 6:00/mile pace but that won't help them much.
I use the same principle when coaching ultrarunners and for my own training, which is why I like to include double or triple marathon weekends. A great option for this is the scenic Tahoe Triple with three marathons over three days around beautiful Lake Tahoe in California. There aren't many opportunities to fit in a marathon on Sat and Sun for double marathon weekends so this year I decided to include the Santa Barbara and Malibu Marathons as a good training weekend since both are in Southern California and are close together.
Even for trail running I find road marathons are very effective at working on speed and improving the ability to judge and maintain pacing. So for road training these are even more effective and I'd strongly recommend a back-to-back weekend like this for those aiming to improve their road marathon time. However, don't just jog the runs at an easy pace - the aim is to get the muscles used to running close to marathon pace.
I acted as an official pacer at Santa Barbara for the sub 3-hr group, knowing that a pace 30 mins off (roughly 1 min/mile) my marathon target time should leave me fresh enough for to run the next day harder. Keeping the mile splits even was a key element here since there's less benefit in going off faster then slowing - it's not a good thing to practice as it's an inefficient (and unenjoyable) way to train and race. Also, it wouldn't be much use for the runners I'm helping to go sub-3. Then the following day I pushed things more at Malibu, still focusing on even splits throughout. Strava files for each race are here for Santa Barbara and Malibu. Both are excellent races that I'd recommend.
There are few training sessions that are more satisfying than this and it can really improve the ability to judge pace and effort in an ultra (or marathon) because of the fact it involves running when tired. Even ultrarunners need marathon speed.