Sent my entry for the p-coy challenge off last week.
Been running in boots once a week for the last few months.
Pulled a groin muscle before christmas which knocked me back & haven't been able to train as hard since. Still not 100% but can do most everything except full sprints & too much ab work.
Finally got a rucksack (karrimor delta 35) last week and ran on Monday with it for the first time with 6.3kg/14lb (and boots). 5.2 miles in 47 minutes and absolutely knackered, but ran all the way including some (though not major) up-hill. No aches or pains & felt ok next day.
Time well within the 11min/mile required to complete p-coy challenge in 1:50. Will stick to 6 miles and slowly increase the weight (1-2kg/week), then when up to 25-30lb start increasing the distance up to 10 miles while trying not to slow down too much as the terrain will slow me down further on the day.
I'm using bags of flour for the weight - 1.5kg bags are 43p each. Heavy but not too compact, and soft. doesn't need too much padding. Just stuffed a light sleeping back into the rucksack and popped the bags of flour down the middle of the sleeping bag. Put paper flour bags in plastic bags & taped up so they don't rip/burst/leak etc.
Rucksack is nice & tight against the back. Jumps up and down a bit but that will probably stop as the weight increases. No aches at all just one of the straps rubbing against my neck but not painful. Might look for a running top with a bit of a turtleneck so it doesn't become an issue.