The Light Blues crashed to a hugely disappointing 3-0 defeat at Ibrox as Terry Butcher's side joined Celtic, St Mirren and Hearts in the last four of the Scottish Communities League Cup.
After beating Motherwell in the previous round, the Gers went into the quarter-final clash in high spirits that they could take another SPL scalp and defy their Irn-Bru Third Division status.
They got what they deserved on the night, however, as Caley Thistle ran out comfortable winners and McCoist knows his side have plenty of work to do if they are to compete at the top level on a regular basis.
He said: "Yes, it probably is a reminder. I am obviously aware of how far we have got to travel.
"I will not make excuses, I will not make excuses for the second half performance and I won't make excuses for the slackness in defending at three goals.
"But, as I have said for a number of weeks now, we have got a long, long, long way to go. Tonight showed that. The league is without doubt the most important thing.
"We have to get out the league. Anything we do in the cups is a bonus.
"That said, it is a big disappointment, but we have a very big game on Saturday against Alloa."
The Gers were never at their best against an in-form Caley side who have impressed in the SPL in recent weeks as they have steadily climbed the table.
After an even first half that saw the visitors take the lead through former Ibrox kid Andrew Shinnie, Butcher's men pulled away after the break as Gary Warren headed in a second and Graeme Shinnie converted from the spot after Anestis Argyriou felled his brother in the penalty box.
McCoist's side were far from good enough as they rarely threatened Antonio Reguero in the Caley Thistle goal and saw their dreams of a Hampden showpiece dashed in emphatic form. Defeat leaves just the William Hill Scottish Cup open for Rangers to make a mark in cups this season, but McCoist had no complaints on a shocking night for the Light Blues.
He said: "I didn't think there was a lot between the teams in the first half, in the second half Inverness were better without doubt and deserve to be in the semi- final more than we do.
"It was an opportunity for us to compete against an SPL team again, which we did do against Motherwell, but the level of intensity we played at was miles short of the level that we played at against Motherwell.
"In the second half, we got what we deserved."
Hearts defeated Dundee United 5-4 on penalties after the game finished 1-1.