If I was routing through Stratford (London) I would probably choose to take the DLR (Docklands Light Railway) between the Elizabeth line station and the international station, rather than walk. It might depend on the day how I was feeling.
I'm not really sure how much time you save taking that Elizabeth line route as opposed to changing at Farringdon onto either Thameslink (in my view easier overall) or the tube to Kings Cross.
If you are landing at 1pm, then you will be arriving at St Pancras around 2.30 pm. If the flight was an hour or more late then on a weekday you are just hitting the start of the afternoon peak. So my own preference would be to join the train at St Pancras (where it starts).
In any event the way the fares structure works it is always cheaper to ticket in two parts-
1) Heathrow to Kings Cross/St Pancras or Stratford via either tube or the Elizabeth Line
2) Kings Cross or Stratford to Canterbury
If you are buying an off peak flexible ticket then the fare is the same now as on the day, so the point of buying now would be for convenience.
What I suggest you do is to buy a Two Together Railcard for £35 (I know you are seniors, but Two Together is better value) to save 1/3 off your rail fares- St Pancras to Canterbury is £43 each (so £86 for the two of you) before the discount and the Elizabeth Line from March 1 is £15.80 each so £31.60 for the two of you before the discount- thus a total of £117.60 for the two of you. To amortise the £35 railcard purchase fee you have to spend £105 to 'break even' which you have. So it doesn't matter if you never use the railcard again- as this is to join the RS tour I assume you may not use it again.
There is an argument (a valid one) for taking the Underground line direct from Heathrow to Kings Cross, but with railcard discount the Elizabeth line is £10.50 vs £5.80 on the tube- so for an extra £4.50 the greater speed and comfort of the Elizabeth Line is possibly worth it.
