I run / jog a relatively short trail, less than a mile, in an effort to maintain fitness. I have been thinking about wearing a pack to increase the effectiveness but my existing packs will not do: to big and bulky, and ride wrong.
Hmm, anything under a mile is really a short distance sprint. How long does it take anyway, 6 or 7 minutes at most if you're in decent shape?
You just don't need to cary any extra gear on a 6-minute sprint and you sure as heck don't need a fancy pack with a hydration system. Not unless you want to do some long distance running, in which case you would do well to invest in a good, tight fitting pack with hip support.
But from what you've said, you want a backpack for added resistance. That's fine if you're in good shape already but you really must develop a solid foundation first. Meaning good endurance (being able to run at least 20 minutes at a fast pace) and having enough lower and upper body strength. Otherwise, running with a heavy pack will ruin your joints and spine in short order.
I've used an old hiking pack (45 liters IIRC) that fits me properly and has a nicely padded hip belt. Make sure it's strong and fits well, everything else is unimportant. You could fill it with sand - your best bet would be to make small sand bags so you can vary the weight as needed.
An even better but expensive alternative would be a weighted vest. It's basically a tight fitting vest with lots of pockets for small weights. It's good stuff because when adjusted properly the weight won't move around at all.
Another alternative still, maybe you can get one of those old, obsolete bulletproof vest with heavy ceramic plates. Easy to make you own weighted vest. Personally I've also had good success with a tactical (LBE) vest. Again, lots of pockets where you can insert small weights. Good for at least up to 30lbs.