What is the most convenient way to get from Heathrow, Terminal 3, to Russel square area, please?
We have 2 duffel bags plus 2 carry-ons, and we are not young any more.
Do we take a train to Kensington first, or do we go straight to the taxi stand?
I see that the Piccadilly Line goes direct from Heathrow to Russell Square, so you can consider whether you're up to that or would prefer a taxi or alternative car service. Others can advise with more recent experience and fare estimates. There's also the fast Heathrow Express train to Paddington, followed by a taxi.
Heathrow express is set up for lots of bags, in a way the tube isn't (either in the carriage, or the long walks/stairs/escalators/most ticket barriers). There is a bit of a walk from the train at Paddington from memory, but grab a trolley or two and you'll be fine. Very easy at the airport end, though a trolley recommended.
My suggestion would be taxi all the way, or Heathrow express + taxi
It is very easy to get the Heathrow Express from Terminal 3. They have trolleys, they have elevators, they have a lot of agents ready to answer questions. If you plan to use an Oyster card you can also take the TFL Heathrow Train which just takes a little bit longer and is way cheaper. Just buy your Oyster card and load it at the airport. More information here.
Depending on what time of day you are arriving you can also consider Just Airports - a private transfer service. The rates can be quite reasonable. You just don't want to do it during rush hour. https://www.justairports.com/
When you say duffel bags, do you mean without wheels? If you each have one piece of wheeled luggage plus a carry-on, that would be perfectly manageable on the tube (I'm 70, and that's what I do). From Terminal 3, you just follow the signs to the Underground - it's all escalators or level access, and all the trains are going in the direction you want. At Russell Square, there are some steps to the lifts which take you up to street level.
Of course, if you have to carry all the luggage, it's a different matter.