Flight from California to Heathrow, Train to Edinburgh

Just curious as to why not flying into Scotland?

Since you'll already be at the airport, why not just fly to Edinburgh and save yourselves the hassle ... and the half day of travel by rail?

You plan works but a lot of effort and you will sleep most of the time on the train. Not the best way to start you jet lag adjustment. To late I know -- why did you not fly directly to Edinburgh?

So long as it is not 21, 24 or 25 June.

On 21 June there is engineering work north of Newcastle so it is a replacement coach from Newcastle to Alnmouth, or a few trains are being diverted north of Newcastle via Carlisle, with a nearly 90 minute delay.

On 24 and 25 June Kings Cross is closed- so you need to travel on Thameslink or Midland Main Line from St Pancras to Bedford, then rail replacement coach to St Neots (a suburban station not normally served by LNER) for your train to Edinburgh.

Otherwise you will have 4 hours to snooze on the train north, and as long as one of you stays awake on the tube train to Kings Cross to ensure you are not over carried to Cockfosters at the north end of the line!
If it suits you go for it.
Can't you find a flight from LHR to EDI, ideally that you could add on to your existing booking as a through booking, to cover any delays on the TA sector.

Flying on to Edinburgh after connecting at Heathrow seems worth considering. I wonder whether you’re flying home from Edinburgh, or with a round trip ticket from London? If the latter, you could return to London on the train. If you’re not in a huge hurry, you could even stop and see (or stay in) York on the way between Edinburgh and London.

To answer your question. Yes, too ambitious. Are you limited in time? You must have had a reason to do train over plane, what was that?

Way too ambitious. Not only the transfers to Kings Cross etc, but what if your plane is delayed or cancelled? You might see about changing your flight to a day earlier, changing your arrival to Edinburgh etc. Yes there will be a cost to those changes. If you book a flight out of LHR to Edinburgh on a separate ticket / carrier and you don't arrive in time you may lose the ticket.

It's perfectly doable but will leave you very tired. If you allow an hour to get clear of LHR and allow an hour to get to King's Cross, plus some delay time, you would maybe aim for 1030 train, arriving about 1500.

If you did fly that's now going to be a separate ticket so you need to leave the time to get the second flight, so you aren't much better off. Plus once you've done the check in time and transfer from EDI to city centre, you probably end up in Edinburgh the same sort of time.

If you can try to amend the flight to one ticket

We did this a few years ago, although we only used the train as far as York. Arrived early A.M., booked a train around 2:00 in case plane was late. Since we had extra time, we visited the British Library which is near Kings Cross and had a quick lunch. There was a place to store luggage close to Kings Cross (expensive, I think like $20 a bag). We had a hard time staying awake on the train which was only a 2 hr. ride, not 4 hr. like it would be to Edinburgh. The suggestion to fly LHR to Edinburgh is a better idea and would probably even save you some time.

Thank you all! We will look into flying to Scotland and save the train for the end of the trip. Much appreciate the help on this forum!
-Cathy