You don't need a rucsac: a decent sized saddle bag will do, and keeps weight (and sweat) off your back. All you need besides what you'd take on a long day ride is shorts and a t-shirt, USBx3 plug, leads, toothbrush/paste, chamois creme, spare socks, possibly spare shorts (or make sure overnight stops have a place where your used then washed shorts will dry). A cycle top if washed will always be dry by the morning (so reports (
@Brandane ) of smells can be grossly exaggerated). With variable weather the bag will need to cope with arm and leg warmers (if not worn) and maybe overboots or at least toe covers).
Most recently worked for this:
South Wales Triangle - Bristol > SevernBr > Aberystwyth > St Ann's Hd > SevernBr
Stayed in YHs and B&B; 675km/8999m in 3 and 1/2 days, but could have done same again ie total a week, no problem.