If you don't yet have a backpack, then borrowing one seems sensible. If you like it, then fine.
My own experience with a large, heavy rucksack was that it was all too easy to fill it with 'just-in-case' and 'backup/spare' and 'luxury' items. The upshot was that it weighed a ton, and walking any distance , especially uphill or over rough terrain, was made much more difficult, and was significantly more tiring than carrying a lighter load.
Do you really need a 'huge' rucksack? If you look at the kit lists on this site, you'll see that you can get by with a vast reduction in the amount of gear you take. Granted, some of those reductions don't come cheap. Do you need a rucksack with a ton of pockets? Again, this might suggest that you are carrying lots of 'just-in-case' items. You also mention having the tent strapped on top of your bag – is it too big (and if it is, it's probably heavy) to fit inside the 'huge' rucksack? And does your rucksack really need to be ultra-tough?
You might eventually consider a pack like (for sample) the Golite Pinnacle. That compresses down to a small size when you want to stabilise the pack with smaller loads, but has 72L of capacity for bulky but light loads (e.g. winter). As a single rucksack for summer and winter conditions, I think it's a good compromise. I own one. Tough, light, versatile, and it even has a pocket :o) (3 if you count the side mesh pockets). It doesn't have bungee compression on the back, but with the available capacity that's not such a big deal.
For summer I use a Vaude Triset 35L Ultralight rucksack, and I'm pleased with that. It weighs the same as the Pinnacle, so certainly isn't the absolute lightest 35L bag available, but it has a good mix of features and an air-space back system that works well in the heat.
Having said all that, if you are just starting out on lightening your load, you might need that big heavy sack to comfortably carry a heavy load, until you've worked out how to get the load down to a level where a lighter rucksack is a comfortable option.
Good luck.
Cheers, Simon