I’m a fit and strong 60 year old.
I'm a tad older than you, a bit over weight (but its 'going down') and walk between 3 & 10 miles most days to keep the Diabetes and High Cholesterol in check.
On longer 'jaunts' I have no difference between a 1-night pack and a 3-night pack except maybe 1 days extra rations.
What I need for 3 nights is pretty much the same as 1-night.
Includes tent, sleeping bag, footprint, sleeping pad/mattress, all cooking equipment, 3 days food, clothes, GPS, VHF Transceiver, washing kit, bits & bobs etc etc.
Carry about 1 days water and a Sawyer filter (or water troughs, pubs, farms etc) for refills.
1-change of clothes is generally sufficient for 1 night or 3 nights.
The only differences are Summer-Pack and Winter-Pack where the Winter pack carries additional gas and thicker / warmer clothes.
Summer Pack weighs just under 10kg and fits into a 25 Litre Pack
Winter pack currently weighs 13.5kg and is in my 65 Litre pack, (but only half fills it - leaving room for additional woolies if necessary..)
If I was spending a few days 'in the high mountains' in the Winter I'd be adding considerably to the weight with additional 'stuff'.