I'd agree with John in general terms. although I'd have put 60lb rather than 70lb as the standard load for a fit adult male. 'SAS stories' and similar nonsense seem to have a fetish about telling people how much they carried, and regularly top out at range of about twice that, so 120-140lb for the super-fit, and I would expect anyone capable of walking the distance to be able to take 20lb-30lb without it making a significant difference.
Lifting above the head is another matter (and sounds like the old D&D strength description wording). I, back in my walking days, used to on occasions carry a pack which I could (just about) heave onto my back, using arm strength, but which I could much more easily put on sitting, or rather, squatting down, and lift by standing using my leg strength. There is no way I could have lifted it above my head, but once on I could carry it for a good while on my back.
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Lifting above the head is another matter (and sounds like the old D&D strength description wording). I, back in my walking days, used to on occasions carry a pack which I could (just about) heave onto my back, using arm strength, but which I could much more easily put on sitting, or rather, squatting down, and lift by standing using my leg strength. There is no way I could have lifted it above my head, but once on I could carry it for a good while on my back.