Sometimes, you have to adapt. I ran an 8 mile race during a winter storm warning in 2015, and I ran a 5K (after a preview of the course) 5 days ago with half the course unplowed for the preview and a few segments still unplowed for the race. It's, um, different.

Much depends on how good your local governments are at snow removal. 9-11 inches would be significant to me. If it isn't plowed well, it's too deep to get a good long run in. The local governments around here are very good at snow removal; but I don't want to be out running in an active snowstorm when the plows are operating. They move along at a pretty good clip, the drivers don't have the best visibility in the world, I might not have a good place to dodge, and who expects to see someone out running in a snowstorm? Once it stops snowing and the plows have made the first pass, I'll run; but I'll choose routes to have a decent chance of moving to the other side of the road if I encounter a plow coming along to clear the shoulder. That last happened . . . Monday, I think.

Sometimes, the conditions are just too awful to do the longest run of marathon training outside. Last February, there was a Saturday with temperatures in the single digits F, 30+ mph winds, and a bit of blowing snow. The 8 mile Freezeroo race was canceled because the local government wouldn't put police officers out to close roads and direct traffic. The local running groups canceled scheduled training runs due to extreme cold. A buddy of mine did 13 miles on a dreadmill. Another runner I know ran 14 miles outside. Several runners I know ran 6 to 8 miles outside. I might have attempted 10 to 13 miles, and expected my water bottles to freeze solid; but my assignment was 22 miles. No way, under those conditions. I ran my 22 miles as 177 laps of an indoor 200 meter track. The following day, with the temperature up to 15º F and winds down to 12-15 mph, I managed a 100 minute run outside - a bit over 12 miles.

Later someone told me I could have done the 22 miles on a treadmill. I just kind of stared at him. The thought had never occurred to me, even though the facility I used also offers treadmills. 22 miles took me almost 3 hours, and I don't think I could stand that long on the dreadmill. I'd defer the long run to a different day before I'd do that. (The happy ending is, that wasn't the only 22 mile run on the schedule. I had a much more pleasant 22 mile run outside later, in warmer weather.)

So what to do with the marathon 3 weeks out and no alternate day available? You do the best you can under the conditions. Maybe settle for 4 x 5 miles with breaks between to swap for warm water bottles and do whatever other heated recovery seems necessary, and count 20 miles with 3 breaks as good enough. Or maybe you end up exhausted and bailing after 3 x 5 miles in the snow. Don't beat yourself up about whatever you ultimately get done. At this point, the mental preparation for the marathon is probably more important than the physical preparation.

@jenring3 to start, let me say that I love winter running. Absolutely love it! Two things though.

1. If you're running through fresh snow, the effort will be exponentially harder and you'll be working muscles differently than you're used to so expect some DOMS afterwards. Because the run is harder, you'll be running slower though your effort will be greater. My suggestion is to figure out how much time your long run would take you on dry pavement and then run by time, not distance. Also try to keep your effort level in line with what you would experience on dry pavement. If you have a heart monitor, run to your normal long run pace. A couple of weeks ago I ran in just 8 inches of fresh, unplowed snow and my pace slowed from 9:00 to 11:00 for the HR I averaged.

2. Depending on the temperature, falling snow could be melting on you as you run eventually soaking your clothing. Last weekend I went out for what I had hoped was going to be a run in the lower teens but the falling snow melted, soaked in, and eventually I got the chills and decided to cut it short at 9 miles. Next time, in those conditions, I'll plan on 2 or 3 smaller loops so I can shed wet outer layers and replace with dry layers.

Other considerations are that you won't be able to see any ice below the snow, or you may miss potholes, sidewalk cracks, and even curbs.

If you want to run, go for it! Just adjust the effort as I described, expecting slower pace and shorter distance (but equal effort), prepare for getting wet, and run cautiously.

It's snowing right now and I hope it's still snowing when I can get out to run later today!