What could have caused blisters when I haven't had them before with these shoes?

[BlackAmericanoNoSugar]

I walk quite a bit, almost always on pavements in my suburban area. I've had these trainers for a couple of years and they've always been fine. Last night I did a 50 minute walk on one of my usual routes and I had a blister on both heels by the end of it. Not bad blisters just loose skin, it hadn't got to the filled with fluid stage, but still sore enough. What could have caused this?

I'm going on holiday soon which will have a good amount of sight-seeing walking (not full hiking) and I was planing to bring sandals and these trainers. I'll bring some heel plasters with me just in case, but I'd rather work out how to prevent blister altogether.

They've stretched with age and are now rubbing?

Either the wrong socks, if they were different - in trainers and walking shoes the socks are more important than the shoes IMO, but more likely, the shoes have had their day.

I don't use walking socks, just regular cotton socks but they are the same as I've always used.

I suppose it's possible that the shoes have aged but I did the same walk on Sunday night with no issues.

It was very slightly warmer than it has been and I was wondering if my feet were a bit swollen. Maybe just enough that I can't see or feel it but enough to change the fit of the shoes?

I get loads of blisters. Besides things to put on your feet beforehand (e.g., Compeed blister stick) or afterwards (Compeed plasters, or similar), you can also do things to the shoes themselves to help.

I recommend Engo patches, which are a very very low friction sort of adhesive patch that you can stick to the shoes in places where they rub. I find this helpful in shoes where I get blisters on the sole, or where edges of straps or buckles or seams rub. They aren't cheap, but worth it. You can get them from Amazon, and you cut them to the size you need. They need to be replaced eventually, but last for quite a while.

Another option is Pellitec pads, again something you can get from amazon. They are thicker, and not frictionless but sort of fabric covered pads. They are very useful for places where the shoes a wearing out - e.g., on the inside heels of trainers, because they sort of pad over the slightly worn bit. They only come in one size and can't be cut, though.

It's also worth trying double layer socks, although I find that the inner layer can easily wear out, get holes or thin patches, etc which can then increase the chances of blisters anyway. If you get double layer liner socks, they are thinner than hiking socks, and so better for walking around cities etc in shoes where you might not want thicker socks.

I think blisters are more likely when your feet are sweaty? At the gym/hiking etc. Seems so to me. I find a bit of talc in socks stops blisters totally.