Rule of thumb, ride at the highest pace you can without needing to breath with your mouth open. If you need to open your mouth to breath in, slow down a bit. As the weeks go by, you will find that your ability to burn fat will increase, your pace will go higher and you won't even be out of breath.
Have to say it again! Thank you SO MUCH for this bit of advice!! Before this, my longest ride was DDO->Terrebonne (57km). Was exhausted by the end of the ride and the rest of my day was shot.
Today: DDO to Oka then ferry to Hudson and back to DDO (paying close attention to my breathing) = 79km. Got back home, made dinner for the family, gave the kids showers and took them on a 4km walk!!! Feel like I could have gotten up to 100km but my ass was sore from the saddle...Working towards my 100km goal, slowly but surely.