First off my answer is panniers. Wearing a 30+ litre rucksack on your back full of shopping as you cycle has got to be some sort of punishment bordering on masochism. Secondly if you find a rucksack is too long in the back for you it probably is. It is like wearing a pair of size 10 shoes if your feet are a size 6. Osprey do female specific rucksacks, which tend to be much shorter in the back as women have shorter backs than men, narrower shoulders and of course different upper body anatomy. Therefore the chest straps of ladies Osrepy packs are a slightly different shape to mens' packs to account for the differences in ladies and mens shapes. Having said all this I still think you should use panniers as carrying a heavy 30 litre rucksack on your back whilst cycling will not be a great experience and likely compromise your cycling performance and your control of your bike It will certainly make you feel very top heavy with a high centre of gravity and may also cause pains in your wrists, shoulders and back more generally.