The journey from Waterloo to St Pancras International on the London Underground typically takes about 15-20 minutes. You'll usually need to change lines at either Embankment or King's Cross St Pancras, depending on your route. Be sure to check for any service updates that might affect travel time.
Take Northern Line northbound to Leicester Square, then change for the Piccadilly Line northbound to get to King's Cross St. Pancras. I recomend that a change at Leicester Square is easier than at Euston.
The Northern Line (Black one)
As there are no direct trains from Heathrow to London Waterloo, it's quicker to travel by underground from Heathrow on the Piccadilly Line to Leicester Square Underground Station and then change onto the Northern Line to London Waterloo Railway Station.
Take the Piccadilly line from Heathrow to King's Cross St Pancras (St Pancras is the Eurostar station).
The most direct path is east to the A201 south, across the Blackfriars Bridge to Southwark and then "The Cut" west to Waterloo Road. It is 6 minutes faster, on average, to use the A4200 due south across the Waterloo Bridge.
Take the Piccadilly underground (tube) line directly to St Pancras from all terminals at Heathrow.
The journey time at morning peak is between 25 and 30 minutes, dependant on whether or not you are prepared to change train enroute.
From Waterloo take the Bakerloo line to Oxford Circus. There transfer to the Victoria line to St. Pancras International rail station. Journey takes about 20 minutes with one change of train. Check the 'London Tube' site for service delays and restrictions.
Yes, Euston is connected to St Pancras via the London Underground. You can take the Northern Line from Euston to King's Cross St Pancras, which is just a short walk away from St Pancras International. The journey between the two stations typically takes about 5-10 minutes. Additionally, they are within walking distance of each other, making it convenient for travelers.
It's complicated as there are no direct trains. Your best bet would be to take a train to Waterloo and then transfer on to the Underground.
couple of hours (3?) if you take the high speed to st pancras
About 1 hour.