Hi Janet,
Yes, taking the C-Train to the Stampede grounds is likely your best bet as what you have been told is true: parking leaves much to be desired :(
Since you're coming in from the Cochrane area, you would drive to Calgary on Highway 1A. Watch for the signs directing you to the Tuscany C-Train station. The Park and Ride area is the name for the parking lots at the C-Train stations. Many parking spots at the Tuscany C-Train station are free, but there are some paid/reserved spots, so don't park in those as you may be ticketed and/or towed.
Adult fares on Calgary Transit are $3.15 CDN per adult. That ticket is good for use on C-Train (light rail transit, aka LRT) as well as the bus system. The ticket is valid for 90 minutes, and you can use it to make as many trips and in whatever direction you like.
If you have a few adults in your party, or if you plan on making a few trips on Calgary Transit, you may wish to purchase a booklet of 10 tickets. There is no cash savings, but it's more convenient than having to buy tickets each time you board.
Last year (2015), they had special day passes specifically during Stampede at a cost of $6.50 per adult ($4.50 per youth). These passes make good financial sense if you're making more than 2 trips per day. Not sure if they will have those passes again this year or not.
Youth from 6 to 17 years are $2.10 per ticket, and children under 6 are free.
It will take you between 25 and 35 minutes on the train to get from Tuscany to Stampede.
The nice thing is that there is a C-Train station right at the Stampede grounds. I do not know if they will do it again this year as they have not made any formal announcements, but last year (2015), the C-Train ran 24 hours per day for the entire Stampede. Granted, between 12:45 am and 5:00 am, it only ran every 30 minutes, but it's still better than not at all :)
Hope you enjoy our city and the Greatest Outdoor Show on Earth!