I mean, 2 days travel coast to coast isn't too far off from doable. Like, fudging the numbers a bit you could blast through the continental US in, if my numbers work right, about a day. 48 hours Cali to DC is a doable trek and if you use the rust bucket to fudge numbers along isn't really the most unreasonable.
Like, the because rust bucket excuse isn't to justify them being physically capable of making it coast to coast in 48 hours, it's to justify the trip not being bloody torture.
Less of an "utter bullshit it would take an idiot to believe" and "more grandpa is a little off", at least until you break out the stopwatches, excel sheets, gps, and corkboard with maps.