Running trainers have a reputation for being the one purchase worth taking seriously, and for once I think that reputation is earned. People treat the rest of their footwear as fashion and their trainers as engineering, and that's broadly the right split. Where I land is simple: spend properly on these, because they're the one item on your feet actually working against impact, repeatedly, for miles at a time.
They're the wrong choice the moment they leave the gym or the pavement and head anywhere a shoe needs to look like a decision rather than an accident. They don't do smart casual, they don't do a wet hike over rocks, and treating them as your only shoe for every kind of movement is how you end up with sore knees and a shoe that's dead within a few months instead of a year. On fit, go a half size up from your usual, because feet swell on a long run and swell more in heat, and there should be a thumb's width of space at the longest toe when you're standing. Width matters more than most people check, a shoe that's snug at the widest part of the foot will tell you about it at mile four, not at the till. Heel slip in the shop usually settles once laced properly, but if your heel is lifting with the laces done up tightly, that's not a shoe that beds in, that's the wrong last for your foot. On colour, light neutrals and greys show grit and mud within a fortnight, which is fine if you don't mind them looking used, but a mid grey or navy with a contrast sole hides the honest reality of a wet winter route far better than white ever will. A well made pair has a secure, well bonded upper that doesn't separate from the sole after a few damp runs, a midsole that still has bounce after a thumb press rather than feeling flat and dead, and a heel counter with proper structure rather than something that collapses the moment you pull the shoe on. Cheap versions cut corners on the outsole rubber first, so check the tread depth and pattern, because a smooth, thin sole is the first thing to go on wet pavement. Seasonally, a more breathable mesh upper is miserable in persistent drizzle and genuinely necessary in July, so if you only own one pair, something in between suits most of the year better than either extreme.
Let them air out properly between runs rather than shoving them straight back in a bag, and never put them through a hot wash or a tumble dryer, the glue and foam will not thank you for either.