You will have been expected to leave a reasonable amount of time from your train arriving at St Pancras to the 0730 departure from Euston. If you train is timetabled to arrive at St Pancras at 07:16, you will be asked to buy a new ticket even if the train you were on is delayed as you would not have left sufficient time to make the connection regardless of if you are tubing or walking it.
As was said before, if your train is timetabled to arrive at St Pancras before 06:55 and you still miss the 07:30 through no fault of your own, the staff will endorse your ticket for travel on the next service. If this does happen, be sure to keep the ticket you used to get to St Pancras as proof that you were delayed by a national rail service.
Try to get to St Pancras for about 06:45 to be on the safe side.