I did 428 miles in the National 24 Hour TT in 2006, but that was split between two calendar days. In a single calendar day, it's probably 250 miles in the National 12 in 2002.
That's similar to the most miles I've ever done in a week! And that was a long time ago. There was a sort of unofficial thing in our club of trying to do 500 miles in a week, but I never quite managed that much. The longest I did involved a couple of days off, a long Sunday club run then a solo ride down to Rosedale Chimney bank to see the Kelloggs Tour (1987 ?), where I racked up about 130 something miles for the day, plus a good few decent rides through the week. Since starting back again in January I've managed a mere flea hop of about 15 miles! On my previous return to the saddle about fifteen years ago (I come and go), I did about 40 miles of railway paths on the MTB (Waskerley down to Cox Green near Sunderland via Hownsgill, Consett, Birtley, Beamish etc. and return. Many years ago (early nineties I think) I did a 162 mile day, which involved a club 150 reliability trial, plus ride to the start & back home after.