I’d reckon on between 50 mins and just over hour.
Route wise I’d probably go Euston to Russell Sq, then Southampton Row to Holborn.
From Holborn along Holborn Viaduct to Holborn Circus, past St Pauls and along Cheapside to Bank.
Once at Bank head up Bishopgate to Liverpool St.
If you’re feeling adventurous you could cut through Smithfield to the back of Liverpool St.