If you can ride the bike until the gas tank is empty, a long trip is just several of these sections put together.
It would be wise to do an all day trip before doing a 2-day trip, then later a 3-day trip. An interstate at 75 mph will be more tiring than a straight highway at 60 mph, and intersections and corners are more risky as you get more tired.
For safety, if you start to feel tired or thirsty or too hot, or you start to get monkey-butt, stop and rest for awhile. The longer the trip, the longer the breaks will probably get toward the end. For a 3-day trip with riding 10 hours a day, I need a couple of hour long breaks during the last 4 hours of riding, now that I am in my 50s.
A 250 will do 1000 miles in 24 hours, but you will likely be more sore and tired afterwards compared to a heavier bike with a higher quality seat. A lot of people add an Airhawk brand seat cushion for longer trips.
In the 1970s, riders with passengers rode 350cc twin cylinder bikes with an 80 mph top speed across the US. Most people think this is way too small now- the weight of a passenger made it more comfortable with the low quality suspensions then.
"Until he reached thirty, Randy felt bad about the fact that he was not socially deft. Now he doesn't give a damn. Pretty soon he'll probably start being proud of it." from Neal Stephenson's Cryptonomicon novel (1999).