Which is longer the grand national r the university boat race?

Both are :-)

The Grand National course is 4 miles, 880 yards - 7,242 metres

The Boat Race course is 4 miles and 374 yards - 6,779 metres

So the Grand National course is longer.

But, as rowing is slower progress than horse-racing The Boat Race takes longer

The current Course Record is 16 mins 19 secs - set by Cambridge in 1998.

The Grand National Course Record is 8mins 47.8 secs set by Mr Frisk 1990.

So, either way, you win your bet.