Bikes and backpacks

First bike ride of the good weather for me... just a short one to start off again. Did a16 miles in an hour so not bad going.  I packed light, just the necessaries in case of emergency...

Water X3 bottles
Bike pump
multitool
maps
puncture repair kit
spare inner tubes X2
First aid kit
camera
phone
keys/wallet etc

The thing is because I packed light, i shoved it all in a rucksack, slung it on my back and off i went.  The thing is after 5 miles my shoulders began to hurt.
Now I'm no stranger to cycling with a backpack.  i cycle to work with one, i cycled from Snowdon to Ben Nevis with a very heavy one and similarly from South to North Wales (no panniers for that one).  but this got me thinking....

I have panniers and will use them for Route 66.  i have previously laughed at people on websites who have said under no circumstances tour with a rucksack and i have done 100 miles in a day wearing one.  Maybe it was because I haven't been on my bike properly for a while, maybe it's cos i'm getting older LOL but the fact remains that 5 miles in, my shoulders hurt, after 10 miles i was shifting position on the bike to get more comfortable and after the 16 i was very glad to get it off and sink into my comfortable sofa.

I was planning for Route 66 to just have a Camelback with keys, money and stuff I didn't want off my person in. But even that is sounding rather heavy at the moment.  I don't particularly want a bar bag but even a small backpack with a litre of water will be heavy after the 3 day of 100 miles.

Still i still have a fair few days to go to think up a solution.  Either that or just put up and shut up, with the thought that I'm going to be soooo knackered that i won't notice aching shoulders at the end of the day.

would be interested to hear your views on this.. ultimately I guess the less kit the better! And Pain is Weakness leaving your body so it's all gotta help! :)