You'd be doing well to make the 7.55am Heathrow Express. To get to Paddington from Euston on the Underground you have to go out of Euston Station, cross the Euston Road and walk along it for a few hundred yards to Euston Square Underground Station. That in itself will probably take 10-15 minutes, especially if you aren't familiar with the terrain. Will you need to by an Underground ticket?

At Paddington, you need to get out of the Underground system (the Hammersmith & City Line doesn't stop at the same place in Paddington Station as the Circle Line), but in either case egress from the Underground system on to the main station concourse will take some minutes.

The 8.10 is much more likely, but it is possible that you won't make it to Paddington until the 8.25. Don't forget this will be the height of the rush hour.

Another possibility, especially given the cost of the aeroplane ticket, would be to bite the bullet and get a taxi from Euston to Paddington: but even then I doubt you'd make the 7.55 Heathrow Express.