Running with A Pack

I've been running to and from work for a few weeks now, which means having to carry a backpack while I run.

Last night, I went for a pack-free run and BOY what a difference!

My speed was fast and I felt GREAT! 

I'd been feeling so lethargic with the pack on, feeling like my fitness must have really slacked off, but it can't weigh more than 5kg (only clothes and work things in).

But then I saw on Biggest Loser an old episode towards the end where they pack all the weight contestants had lost over the months and made them hike up a hill!

So I'm going to try adding more weights to my pack gradually and see if my fitness responds!  I'd love to be able to run the speeds I did last night with no pack!

Any suggestions for weights that wont jiggle around too much in the pack?  Was thinking maybe 2lt bottles of water?

PS - I dont have any hand weights and I'm moving house soon so I dont want to invest in them just yet.  

I run with a pack also - not as big/heavy as yours, however - despite how tight the bag is around my waist/back, the weight still jiggles when I run and I don't really like that too much. Instead of something small and dense, maybe you can put a thick book in it that you can strap and station it close to your back? I'm not sure what kind of back pack you have though.

Diving weights might work - they come in a pouch shape, so they should settle into the bottom of the bag or pocket and not move around much.

I dont run with or without a pack, but when i walk home from the gym, it weighs me down as well. Try putting in a pair of shoes, or a water bottle, and packing a towel or two around it to hold it in place.

Unless it's a six pack and I'm running home to drink it, there is no running with extra weight for me

This is just my opinion but I don't think running with a heavier pack will make you faster.  I think that it will just make you slower while you are carrying it.  Speed drills and other forms of interval training will make you faster with or without a pack.