Out here in northern Wyoming the Search & Rescue are always going out for people who did not prepare adequately or allow for the unexpected. You can expect big thunderstorms to crop up most afternoons in the Plains. Diaries over at Crazyguy mention them often. It's not a great idea to be riding thru lightning when you are on a hunk of metal; however, if you have an inflexible schedule pushing you - it is more likely that you would take that chance. Just saying - - -
Same goes for riding when you are tired, not feeling well, into headwinds for mile after mile. Those are the situations where accidents are more likely to occur. Years ago there was this guy who started to ride with me halfway across the country - I didn't want him with me - but he tagged along. He also insisted on riding on into a brutal headwind. He wove out in front of a car and got hit. He was lucky he escaped with only a broken arm.
My math comes out to 82 miles per day at 54 days. With one day off per week (or two half days because of weather, etc.) that's 9x6=54 - or nine weeks. Another option is to cut east from Osceola Wisconsin to Manitowoc - take the ferry across Lake Michigan - cut across Michigan to Port Huron - then cut across Ontario and get back on the Northern Tier at Niagara Falls. That saves about 500 miles.