The train takes about 45 minutes from Liverpool Street, or 35 minutes from Tottenham Hale which is on the Victoria Line. You could get the Piccadilly line tube from Leicester Sq to Finsbury Park, cross the platform and catch the Victoria Line where it is two stops to Tottenham Hale. This should be about 30 minute journey.

The train runs every 15 minutes for most of the day.

Hope this helps.