Something which fits rather better than it sounds like those boots did! I can't tell you what specifically because it is all about the fit here.
Not that its easy - long distance walks like that are rather good at amplifying any small fit issues into non trivial problems. Especially if your feet aren't used to it.
(I've definetly done it myself at times.).
Especially on the balls of your feet, because a tiny bit of grit or something getting in can do that quite easily.
The toe nails could be two things. Either there's just not enough room in the toe box or they're not locked down tightly enough to your foot so you're slipping into the front on up/downhill bits. Other dangers are socks bunching up after a fair bit of wear (get them tight fitting if possible), and your heel rubbing if there's any heel lift.
The blister plaster things do definetly seem to help. But mostly get some properly fitted footwear 