The physics in cycling

OK, please bear with me as I lead up to how I got onto my torts on
physics in cycling.

I have just walked in and I am still sweating after riding the 14kms
back to my place via my usual slightly longer route and it has taken me
34mins, which is about average over the 3-odd years that I have been
riding that piece of tarmac.

So what, I hear you say?

Well, this arvo, on the way to the Eastern Suburbs, I found some canvas
which someone had tossed out during one of those garbage days. This is
not just any bit of canvas but seems to be something for a small
truck/trailer/ute. It has eyelets and ropes and good stitching so I am
assuming there is a rip somewhere, but I dunno cos it is all neatly
folded up in a bundle weighing about 8kgs and I have not opened and
spread the whole thing out yet.

Anyway, I decided I wanted this bit of road find (I am using the
"definition" at this link - http://www.sheldonbrown.com/eagle.html), so
I lashed this bundle of canvas (approx dimensions 30cm x 20cm x 45cm
unlashed) to my backpack using a bit of rope which I always carry around
with me in my backpack - bits of rope are often very useful, I find.

So, I am now carrying an extra 8kgs - never mind what the total weight
of backpack and bundle of canvas is (it's 12kgs if you must know - I
weighed it using some electronic scales I have access to at this place I
know - ok ok ok 11.8kgs, for crying out loud!!! One decimal place is all
I am gonna give yer, alright???).

Now, I would have tort that carrying the extra weight (and yes, it
pulled a bit harder on the shoulder straps of my backpack) would mean
pedalling my bike would be more difficult and becos I am not that
strong, my time for the 14kms would be around the 40-45min mark.

You can imagine my surprise when I clocked 34mins!

As the good Professor would say, "Why is this so?"

Here's my guess.

Once I have overcome the starting friction from the additional weight,
my rolling momentum ...... eeerrr... keeps me rolling.

Therefore, the additional effort required is not really that much more.

Also, some of the slight downhill inclines along my route generate
faster (ok ok ok slightly faster?) speeds due to the additional weight I
am carrying.

In the uphill sections, I do feel the additional effort (a bit) if I
remain seated but I would "stand" on the pedals and drive down with my
hips whilst trying to balance the backpack in line with the downward
direction of my driving action. In other words, trying to use the weight
of the backpack to increase my downward force on the pedals, hopefully
making up for the additional load I am carrying (well, that was my
rationale and I have no idea why I tort it should work other than simple
physics).

I dunno if I am making sense here.

Maybe there are others here in this newsgroup with a better
understanding of cycling AND physics who might offer a better
explanation cos I am kinda fresh out of explanations and I am still
trying to satisfy myself why I have done a bog standard average time
when I should not have.

PS - No changes to bike (the Cannondale). Bike was as it was from Monday.

PSS - No changes to my physical self other than my normal diet on a
normal Thursday unless you wanna count the bacon and eggs I had as
dinner I have bacon and eggs and toast regularly for dinner (with a
salad+fetta+olives).