The time on an advance ticket will be for the main line section, i.e. from Euston.
You must travel on the specific train from Euston which the ticket is for; if you do not you will be treated as if you have no ticket at all, so allow a lot of extra time to reach Euston.
If you are not booking weeks ahead, you may well find that the advance ticket is no cheaper than, and because of the way the price quotas work, they can even be more expensive than a normal walk up ticket that can be used on any train ( subject to time restrictions, which vary from route to route ).
Without knowing the specific route and intended time of day for your travel, I can't give more specific advice than that I'm afraid.