The new-look Light Blues needed a last-minute goal to earn a 2-2 draw at Peterhead in their first outing in the Third Division on Saturday, while only an extra-time winner at Brechin earned Gers a right of passage through to the second round of the Ramsdens Cup the week before.
Now Johnston, who starred for the Ibrox side between 2000 and 2001, has admitted that particularly away from home Ally McCoist's men are going to face some gruelling tests of character, as they found out at Balmoor on Saturday.
As player/manager of Second Division Queen of the South, Johnston knows all about the trials and tribulations awaiting McCoist's men and has warned his former club that there will be more lower league ambushes awaiting them.
The 38-year-old said: "It is not going to be just as easy as people think it may be for Rangers to march through the SFL with three back-to-back titles.
"There are a lot of factors that will make that difficult and I think that the Peterhead game will have given Rangers a wake-up call in that respect.
"Firstly I would say that you can't underestimate just how much a team will raise their game when they have Rangers coming to their place.
"Balmoor Stadium was full to capacity on Saturday and you are talking about the Peterhead lads treating that one as their cup final.
"Complacency can also be a problem for Rangers and if that was a factor on Saturday then they will know now this is not going to be a canter through the SFL."
The Queens boss added: "I believe it will be a test of character at times for Rangers to get the whole way back up to the SPL but for me quality always comes out on top in the end.
"Rangers are a massive club who will always attract quality players as they have just proven with Ian Black, Dean Shiels and Francisco Sandaza all signed and sealed.
"It is not going to be easy and they will definitely drop more points over the course of the season but Rangers are assembling just too much firepower to fail."
With a season of planes, trains and automobiles awaiting the Ibrox men as they face treks to the most far-flung and unglamorous venues in the Scottish game, Johnston believes their biggest test still lies ahead of them.
The former Scotland star said: "I think one of the other problems Rangers are going to face will come in the winter months.
"They will be going to wee grounds where the pitches are not particularly good in any respect and if the weather is poor the surfaces will have deteriorated further and that will be a great leveller.
"It is when you are playing at grounds like Borough Briggs up at Elgin or Shielfield down at Berwick in midweek and the rain is lashing down and the winds howling a gale that they will need to show character.
"The other thing here is that Rangers will be used to games going ahead even in poor weather because SPL clubs have measures in place.
"You won't get that in the Third Division and they could be on the wrong end of a good few postponements."