Your first long ride should not be your first ride of the season.
Before attempting a goal distance, get your bicycle out early and go on some leisurely rides of 50%-60% of your planned distance. For example, a few rides of 50-100 km should be completed prior to a 200 km ride and at least one ride of 125-175 km is recommended. For a 100 km Populaire, ride a few 25 and 50 km routes and at least one 75 km route.
Be sure your bicycle is in top condition. Have it set up to fit you properly (riding 100 km on a bike that does not fit you can be an unhappy experience). Make sure your bike is dependable and comfortable: the basics include wheels that are true, a good supple saddle at the correct height, and an efficient posture as given by the proper size frame and stem extension.
Experience will be your best guide as to what to bring and how to ride a Brevet or Randonee. Before you get this experience the best course of action is to talk to and ride with more experienced members of the club and try out their suggestions. Many things are personal preference. Err on the side of caution and bring too much food and too many clothes on your first few rides...
As many rides go into the foothills and/or mountains, you cannot depend on the weather (or, you can depend on it changing). Be prepared for rapid weather changes that are characteristic of the Rockies. You may experience all four seasons in one day + night!
It may be feasible to leave some of these at home depending upon the expected weather and ride-length, but you should at least consider all of the following. Bear in mind that many people have different lists -- if you get cold easily or prefer different sorts of clothes, by all means use what works for you. As you gain experience on the shorter rides, you will know more about what you're comfortable taking on the longer rides.
Shorter rides of 200km or 300km can often be completed in the daylight in summer (note that 300km rides or longer require participants to have front and rear lights, even if you expect to finish in daylight).
This should be the bare minimum you should consider carrying. Many people carry much more (e.g. warm fleece jackets on longer rides, change of jersey/shorts for 600km and greater rides, etcetera).
The only thing you need in this list is the brevet card.
On a ride, you will burn up many times the calories you would on a normal day, so it will be important to eat continuously. You may choose to carry food in your panniers, but if you are concerned about weight, you may choose to bring only money. The route maps will display the towns so you will know where food can be purchased (double-check which controls have food options and when they close, however).
The food given out at selected checkpoints (on supported rides) will not be enough to fuel your body for the entire ride. (Most Alberta rides are not supported).
It is advisable to carry some food items with you to prevent "hitting the wall" (depletion of body glycogen) or hypoglycemia (low blood sugar, which can cause hallucinations and/or blackouts).
Suggested items are:
If you're inexperienced with on-bike eating, the best thing is to ask other experienced rides what they do and experiment yourself. Different things work well for different people.
Eat a large quantity of complex carbohydrates for two days prior to and after a ride (e.g., whole-grain foods, starches, fruit and vegetables). It is important to eat carbohydrates following a ride in order to replenish depleted glycogen. Carbo-loading is a process that allows for an extra buildup of muscle glycogen, which itself limits how long you can undergo endurance exercise.
A combination of proper exercise and diet prior to a long ride is required to achieve carbo-loading. See appropriate literature to learn how to achieve this state.
Everyone has their own morning routine before long rides, but obviously eating a hearty breakfast (stay away from greasy foods) is a good idea.
Avoid excessive meals (especially high in protein and fats, e.g., steak and ice cream) before and during a ride. (Although many people swear by a mid-ride ice-cream or chocolate milk).
During a ride, it is better to eat small quantities on a regular basis (every half hour, for example).
It is important to replenish your fluids during exercise. Ensure that you are not dehydrated before the ride. If your urine is yellow, drink, drink and be merry! Water is the best fluid to consume (many people add "hydration powder" such as plain salt or other "electrolyte-replacement" mixes like Gatorade).
Stay away from carbonated and high sugar or salt content beverages. Acidic fruit juices (citrus, tomato) tend to cause discomfort.
You should carry at least two water bottles on your bike and drink one every half hour, even if you are not thirsty. Sip regularly rather than gulp. Fill your bottles at every opportunity (areas will be highlighted on route maps). On some rides, you may find it necessary to obtain water from creeks.
For this purpose, carry purification tablets to avoid water contamination. Many riders feel comfortable drinking water from the streams in the mountains, but they can of course be contaminated.
Remember, if you become hungry or thirsty on a ride, it is too late to replenish energy or fluids!