I'll be flying into Heathrow on a Saturday morning in the late summer, arriving 9:30 a.m., and heading north out of Euston Station. For buying train tickets in advance, what is a reasonable time to schedule my departure from Euston? Basically the question is... How much time to allow for going through immigration/customs, catching the express train to Paddington, and taking the Tube to Euston?

For me, the holdup at Heathrow lately has been luggage!

I assume an hour to get through immigration/customs and get luggage and then 90 minutes to walk to the tube/train and then travel to Euston.

That is assuming everything goes well. To be safe, if I landed at 9:30 I would probably not book anything earlier than 13:30 and just get something to eat at Euston station for lunch while waiting if I am early. But I am pretty cautious and like to leave lots of room for glitches (like getting lost, walking through stations, late arrival of plane, long line at immigration, luggage taking an hour to come out...)

Thanks! Good to know about delays in baggage claim. More motivation for me to do carry-on only, which I'm getting pretty good at. The bulk of my trip is a hike across northern England, Irish Sea to North Sea, and I've done comparable international hiking trips with just my carry-on.

Even with carry-ons only in my experience you need to consider the following along with what I think are Jamikins' excellent recommendations.

Where your arrival gate is and the walk which can seem like "forever" from some of the Heathrow gates to Immigration/Customs and being"something" like FastTrack does not make it any shorter. Your hiking experience will help.

IME “they” get you one way or another. If your flight is early, there’s no free gate (gates vary unlike the US), if the flight is early and there is a free gate, there’s no one to operate the bridge, if all that’s ok there’s a hike to immigration, if there’s no hike to immigration there’s a huge line, if there’s no huge line you wait ages for luggage. So yes, allow plenty of time between flight & train although 4 hours seems to be over cautious IMO based passing thru LHR on a regular basis.

You might have to bear the cost of adding in some flexibility for your train from Euston: since you're arriving at Euston well after 0930, look for an offpeak ticket, which should be valid on more than one train.

Have you check on Naional Express buses right from Heathrow to where you are heading - all in all would be more convenient and quicker if not going too far?

"since you're arriving at Euston well after 0930, look for an offpeak ticket, which should be valid on more than one train."

Shouldn't OP's train be off peak no matter what since the arrival is on a Saturday?

Yes, hadn't picked up on that. So no need to worry about being in time to catch a particular train, but that can always be checked on National Rail Enquiries - Official source for UK train times and timetables