One person runs a 4:20 marathon and another runs a 2:10 marathon. A four hour race and a two hour race are vastly different. Where am I headed with this? People need to train differently. A four or five hour race sounds more like an ultramarathon than a marathon. Ok here is good ultra training: super long long runs up to 70% of the race distance (huh, that's about an 18 miler for a marathon). Running miles just to get used to running and building an aerobic base. Any speed work at all is nice and I guess helps a little but the main thing are those long runs. The thing is you will be running the race at the same pace as your easy runs so every mile is pretty race specific. Good two hour race training: lots of intense pace workouts around goal pace, slower and faster as well. Say an hour at goal pace. 30 minutes somewhat faster than goal pace or an hour and a half slightly slower than goal pace, but faster than an easy pace. Still very aerobic runs but faster than for the ultra race. For a two hour race there is a good chance that you will do a 2.5-3 hour long run that covers the same distance as your race, maybe even farther.
The problem is I read about people trying to survive a 15 mile run or a marathon but I don't know what to say because I've only run one ultra. I also mapped out my own marathon and ran it just to see what it was like. The thing is I am in good enough shape that I felt great and was able to turn in like 15 miles over two runs or something the next day. So for me a 20 miler is not much over two hours. But I can't tell a five hour marathoner to go out on a 20 mile run because they would be out there for four hours and I would hate to tell someone to go out and run for four hours to get ready for a marathon. Because I know from experience that if I was to run for four hours I would be so exhausted I would break a rib or something later that day.
So keep in mind what you are training for. Finishing the race, qualifying for Boston or something even more difficult. Your training needs to reflect your fitness and your goals. Regardless, get out there in the blizzard and get the work done!