"We will probably not have a car, as we will come by plane."
Unless you are arriving and departing from the Mont Tremblant International Airport (mostly private aircraft, but it does have very limited scheduled passenger service from Toronto and, in ski season, from Newark), you might want to consider renting a car and driving from Montreal or Ottawa airports, depending upon which you will be using.
Perhaps there will be special arrangements during the Ironman, but summer season ground transportation from Montreal's Trudeau International Airport normally is limited to sedans, limos and vans. Of the most convenient options, the least expensive would be a sedan and driver for about $610 round trip, plus tip. A car rental for 4 days should be much less than that. Having a car also would enable you to explore the immediate area a bit, try a non-resort restaurant or two and, (if you wish to cook) to buy food in one of the good supermarkets, all of which are about a 10-12 minute drive from the resort.
[There is a regular bus from downtown Montreal to Mont Tremblant. It would be cheaper, but time consuming and not really convenient, for your purposes. You first would have to get from the airport to downtown, then mesh your arrival time with the Galland Bus schedule. They have 3 departures daily from Montreal (7:30 a.m., 1:30 p.m. and 5:30 p.m.). Travel time for the bus is 3.5 to 4 hours and only the 1:30 p.m. bus goes directly to the resort. The other two require a transfer to a Mont Tremblant city bus.]
As bogwoppit said, without a car you also can easily just stay in the resort and eat in the resort restaurants, if that is your preference. Taxis and the Mont Tremblant city bus would give you some flexibility to see, shop and eat in nearby villages.
Notice that you are from Kennett Square. I am originally from Media, so hope that your first visit to Mont Tremblant will be the start of frequent vacations there. It is a beautiful place.